After seeing this brought up over and over again, I'm tired of hearing this argument as one to buy something over another. So I will post some information so I can stop answering this in posts 
Plus it was educational for me to get all of it together in my head as well.
We know that specwise the aluminum and poly macbook lowend models are the same now except for ram. Many claim the superiority of DDR3 as their reasoning for choosing one over the other. Lets examine the issue
The polymac has 2gb of DDR2 @667mhz much like it's predecessors. The aluminum mac has 2gb of DDR3 @1067mhz. At first glance you might say WOW! 1067 is alot more megahertz than 667! I gotta have that.
This doesn't figure in 3 things.
1. DDR3 increases speed at the penalty of higher latency. In real life terms, lets simplify it alot and say that DDR2 and DDR3 are cars, and that the top speed of DDR2 in this case is 66 mph and the top speed of DDR3 is 106mph. But DDR2 has lower latency than DDR3, so lets say DDR2 only has to drive 5 miles and DDR3 has to drive 8 miles. So if the speed were the same it would take DDR3 longer cause it has a longer way to drive. DDR3 makes up for it by driving faster, and as you can see the faster you can get DDR3 the less difference the latency makes (ie that 3 miles makes less over all difference when you drive 160mph ie DDR3 1600mhz (no not availible on macbook)) What this does mean though is that at lower speeds of DDR3, there isn't much benefit as the extra latency makes the performance very similar.
2. According to others you can upgrade your ram to DDR2 800mhz
https://forums.macrumors.com/threads/651330/
http://discussions.apple.com/thread.jspa?threadID=1877953&tstart=0
Now let's go back to point 1. Now if DDR2 can go 80mph over 5 miles vs DDR3 going 106mph over 8 miles, there's probably even less of a difference.
3. The last and most important thing to remember is that computer performance is tied to all the hardware, and is bottlenecked by some things far more than others. In some benchmarking between DDR2 800mhz and DDR3 1067mhz on a PC with virtually identical hardware otherwise, the bandwidth increased from 6146 to 6613... or about 10%. Which sounds alot faster. But then they performed other benchmarks, such as super pi, which showed that
the DDR2 800mhz finished in 46.08 seconds and the DDR3 1067mhz finished in 45.11 seconds. The difference is 0.97 seconds or a whopping 2.1% performance boost.
http://www.anandtech.com/memory/showdoc.aspx?i=2989&p=8
To further illustrate this here are some benchmarks with the old polymacbook vs the new aluminum macbook
http://eshop.macsales.com/shop/Memory_Benchmark/Apple_MacBook/
To better demostrate apples to apples, (no pun intended we'll look at the 2.4 ghz macbook aluminum vs the 2.4ghz polycarbonate) in photoshop CS3
Lower time is better
Macbook Aluminum 2GB ram 99.34
Polucarbonate macbook 2GB ram 107.63
which shows that the macbook aluminum is faster 1067 vs 667 here.
But what happens when you upgrade to a 320gb 7200 rpm drive?
Macbook aluminum 2gb ram stock 99.34
Macbook aluminum 2gb ram 320gb 92.28
how about when we upgrade the ram?
Macbook aluminum 4gb ram stock 79.06
Macbook aluminum 4gb ram 320gb 77.19
Polycarbonate macbook 4gb 78.41
The gap in performance closes to basically nothing. But it does show that upgrading the ram and the hard drive will result in far more real world benefit.
So ideally, you could get a new nvidia whitebook 999, 4gb of DDR2 800mhz ram for 47$, and a 320gb 7200 rpm drive for about 70$, you would get a total of ~1120ish for a new laptop with 4gb and 320gb 7200rpm drive... plus you have an extra 120gb drive for backups now. You could probably even ebay your old 2gb ram for 20$ or give it to a old polycarbonate mac user
The final laptop you would have would be at least in using photoshop, 37% faster at these tasks than the stock 999 notebook, 27% faster than the stock aluminum macbook for 180 dollars less.
You could pimp out an aluminum macbook as well. Which would be 1299 + 70$ for hard drive... and 70$ for ram = 1440$ which would be according th benchmarks ~1% faster. Granted if you could get a refurb, it would only be 1099+140 = 1240, which is only 120$ more. But that is the price you pay for the aluminum enclosure.
In the interest of full disclosure I do own a macbook aluminum. But I got it before they updated the poly macbook. I absolutely despise integrated intel graphics. My choice was clear then, not so much now.
Plus it was educational for me to get all of it together in my head as well.We know that specwise the aluminum and poly macbook lowend models are the same now except for ram. Many claim the superiority of DDR3 as their reasoning for choosing one over the other. Lets examine the issue
The polymac has 2gb of DDR2 @667mhz much like it's predecessors. The aluminum mac has 2gb of DDR3 @1067mhz. At first glance you might say WOW! 1067 is alot more megahertz than 667! I gotta have that.
This doesn't figure in 3 things.
1. DDR3 increases speed at the penalty of higher latency. In real life terms, lets simplify it alot and say that DDR2 and DDR3 are cars, and that the top speed of DDR2 in this case is 66 mph and the top speed of DDR3 is 106mph. But DDR2 has lower latency than DDR3, so lets say DDR2 only has to drive 5 miles and DDR3 has to drive 8 miles. So if the speed were the same it would take DDR3 longer cause it has a longer way to drive. DDR3 makes up for it by driving faster, and as you can see the faster you can get DDR3 the less difference the latency makes (ie that 3 miles makes less over all difference when you drive 160mph ie DDR3 1600mhz (no not availible on macbook)) What this does mean though is that at lower speeds of DDR3, there isn't much benefit as the extra latency makes the performance very similar.
2. According to others you can upgrade your ram to DDR2 800mhz
https://forums.macrumors.com/threads/651330/
http://discussions.apple.com/thread.jspa?threadID=1877953&tstart=0
Now let's go back to point 1. Now if DDR2 can go 80mph over 5 miles vs DDR3 going 106mph over 8 miles, there's probably even less of a difference.
3. The last and most important thing to remember is that computer performance is tied to all the hardware, and is bottlenecked by some things far more than others. In some benchmarking between DDR2 800mhz and DDR3 1067mhz on a PC with virtually identical hardware otherwise, the bandwidth increased from 6146 to 6613... or about 10%. Which sounds alot faster. But then they performed other benchmarks, such as super pi, which showed that
the DDR2 800mhz finished in 46.08 seconds and the DDR3 1067mhz finished in 45.11 seconds. The difference is 0.97 seconds or a whopping 2.1% performance boost.
http://www.anandtech.com/memory/showdoc.aspx?i=2989&p=8
To further illustrate this here are some benchmarks with the old polymacbook vs the new aluminum macbook
http://eshop.macsales.com/shop/Memory_Benchmark/Apple_MacBook/
To better demostrate apples to apples, (no pun intended we'll look at the 2.4 ghz macbook aluminum vs the 2.4ghz polycarbonate) in photoshop CS3
Lower time is better
Macbook Aluminum 2GB ram 99.34
Polucarbonate macbook 2GB ram 107.63
which shows that the macbook aluminum is faster 1067 vs 667 here.
But what happens when you upgrade to a 320gb 7200 rpm drive?
Macbook aluminum 2gb ram stock 99.34
Macbook aluminum 2gb ram 320gb 92.28
how about when we upgrade the ram?
Macbook aluminum 4gb ram stock 79.06
Macbook aluminum 4gb ram 320gb 77.19
Polycarbonate macbook 4gb 78.41
The gap in performance closes to basically nothing. But it does show that upgrading the ram and the hard drive will result in far more real world benefit.
So ideally, you could get a new nvidia whitebook 999, 4gb of DDR2 800mhz ram for 47$, and a 320gb 7200 rpm drive for about 70$, you would get a total of ~1120ish for a new laptop with 4gb and 320gb 7200rpm drive... plus you have an extra 120gb drive for backups now. You could probably even ebay your old 2gb ram for 20$ or give it to a old polycarbonate mac user
The final laptop you would have would be at least in using photoshop, 37% faster at these tasks than the stock 999 notebook, 27% faster than the stock aluminum macbook for 180 dollars less.
You could pimp out an aluminum macbook as well. Which would be 1299 + 70$ for hard drive... and 70$ for ram = 1440$ which would be according th benchmarks ~1% faster. Granted if you could get a refurb, it would only be 1099+140 = 1240, which is only 120$ more. But that is the price you pay for the aluminum enclosure.
In the interest of full disclosure I do own a macbook aluminum. But I got it before they updated the poly macbook. I absolutely despise integrated intel graphics. My choice was clear then, not so much now.
