The difference between CD and C2D is huge I don't know about on mac but on a PC it's insane most single core Athalon 64 can out preform most CD even the extreme CD chip CD also uses tons of wattage and is hotter than a waffle iron in a Denny's. also CD has no 64bit OS support witch means no upgrading to leopard unless they make a 32bit leopard OS and from what I have read and understand it's going to be strictly 64 bit. C2D uses low wattage and has top performance in the PC chip industry now also doesn't get hot. If I where you I would go either C2D or one of the older Motorola's chips.
The difference between CD and C2D is NOT huge.
See the benchmarks here for the CD 2GHZ Black Macbook vs. the C2D 2GHZ White Macbook:
http://www.macworld.com/2006/11/reviews/13inmacbookcore2/index.php
I would call those improvements marginal, at best.
Also, to say that Leopard is only going to be 64 bit only is a ridiculous assertion. Do you honestly think Apple would not allow people who bought a Mac less than a year ago not upgrade to Leopard?
Will a C2D be faster than a CD? Yes, but only slightly.
Is it worth the extra money? Maybe, that is up to you. For the extra $300, you also get more RAM, a bigger HDD, and a Superdrive, along with the slightly better chip.
I would probably go for the C2D myself, but not because the different chip. It would be because of everything else. If you don't want/need a Superdrive and you can get better prices for RAM and HDD from a 3rd Party, you man not want to spend the extra $300.
Is it possible to get 512MB of RAM and a 80 GB 5200 RPM SATA HDD for less than $300?
From Crucial.com, you can get:
512 MB RAM for $52.99
1 GB RAM for $95.99
From Newegg.com, you can get:
80 GB SATA HDD for as low as $59.99
And this is for drives made by Samsung, Fujitsu, etc. Not some no name company.
So, if you went for the CD and upgraded yourself, you can get a machine that is nearly as fast as the C2D for about $200 less. You just won't get a Superdrive.
You could also sell your old HDD and RAM if you wanted and save even more money.
Replacing the HDD on a Macbook is something you can easily do yourself.