Honestly if they move to the Core i7 architecture, the new macs will dominate. It will be like when they went to Core 2 Duo. Does that mean that your machine will be slow and horrible? No. Buy if you have a need now, or wait and get the new architecture. The past few years have been mostly incremental in their increases on the Core 2 line. Then again, the new macs will be the first of their line and might have issues, ie like when apple release the unibody macbook, then revised it a few months later to add back firewire, built in batt and sd card slot etc etc. So it's kind of a coin toss either way. As for buying the higher end one in hopes of future proofing, I have to say that's a pretty stupid argument if you look at it from an economics perspective.
For instance look at how I screwed up lol.
I purchased a 13inch low end unibody macbook in jan for 1200$. It ran fine, but I grew jealous of the light up keyboard and faster clock (stupid in retrospect) I bought my own 4gb of ram and 320gb 7200 rpm drive.) The machine did make a horrible static noise when I used my shure SE530s in them so they were going to exchange it for me. (about 1 week after purchase)
So I got the 13 inch Higher speced unibody macbook 2.4ghz in January. At the low low price of 1500 total.. Because the 400mhz seemed alot faster, and clock for clock it was 20% faster. To be honest, there wasn't alot of practical increase in performance. (Really unnoticable unless I was encoding and actively counting the time, a 30 min h264 file takes 1 hour on the 2, and 50 mins on the 2.4 ghz lol. still a heck of a long time.
Fast forward to june and apple announces the 13 Macbook pro for much less lol, with a longer battery and better screen. Eventually I sold my UMB for about 900$ (after ebay and paypal fees) and I bought a 2.26 umbp for 1099 (with education discount) during the ipod back to school special. I ebayed the itouch for 150$ after fees and such. And after taxes I was net negative 100$ But I had a new 2.26 umbp. I did some testing and the 2.26 cpu was actually faster than the 2.4 in the umb in about 1/2 of all tests. (later core revision) So for the most part I ended up with a better spec'ed machine.
Now if I stuck with the 2.0ghz, I probably would have only gotten about 750-800$ instead of 900$... But I would have spent 300$ less initially.
The macbook pro 13 2.26 and 2.53 ghz models are 300$ in difference. More ram for sure, but that's cheap (70$) and Larger HD (which many people will swap out anyways, I've never actually booted the 160gb drive that came with it. I just stuck in my old drive I had upgraded in my umb) The clock speed difference is now 10% (real life performace difference much less)
A price difference of about 200$ If you save that now sell your macbook when a refresh occurs, and apply that amount to buying a new low end one then, you'll get a new shiny machine (with a new warranty even) and the new low end machine will likely be faster than the high end model of the previous model, and have new features.
Also to be honest even if you plan on keeping the laptop, the increase in clock speed will not keep your laptop from being slow, or unable to run something new. If the program ran way to slow on the lower spec machine, the few percent increase on the higher spec one won't make it run acceptably.
All companies will charge you mucho $ for the incremental increase in performance. (The funny part is that according to intel, they charge the same price for a 2.26ghz processor as the 2.53 ghz processor)
http://ark.intel.com/ProductCollection.aspx?brand=34530
I picked the 25w TDP 1066 mhz ones for comparison
P8400 2.26ghz 3mb cache = 209$
P8600 2.4ghz 3mb cache = 209$
p8700 2.53ghz 3mb cache = 209$
P8800 2.66ghz 3mb cache = 241$
P9700 2.8ghz 6mb cache = 348$
I know the last one has more cache, but look at the increase. is the p9700 75% faster than the p8700? probably not. but it costs 75% more for probably ~15% cpu speed increase and ~5% computer performance.
But if you have unlimited budget, by all means go ahead
