I would go with the Gigabyte 965P S3. It's about $30 cheaper than the DS3 and the only difference is the lack of solid caps.
Also that memory is overkill. If you will not be overclocking get some 533 ram for about $110 (A-Bit is good) or for a mild overclock (like a 333mhz FSB with 1:1 ratio) you can get away with some 667 memory for a little more than the 533. If you are doing some 400+ FSB overclocking then you can get 800 memory
As for the case and power supply I have heard horrible things about aspire. I would reccomend this case and this powersupply. You can get both for about $100.
Finally the graphics card. With the money saved on all the other components I would upgrade him to a X1900GT or if you are going to be waiting a few weeks a X1950pro (so long as they start out at their suggested retail price of $200)
No one needs >800mhz ram. And besides he's trying to stay within $900. Better choice is to downgrade the ram speed to 667 get the S3, and get a better video card. That's gonna give him the best performance for a gaming rig anyway.
his 7600GS is ok, maybe a bit more to get the 7600GT. I would stick with the ram and the DS3. It's a a decent price and will pretty much guarantee him a 400FSB OC given that the mobo decides to be nice to him without much tweaks.
So does OP decide on OCing?
chadatog has his points too, saving ~50 could get you a way better vid card.
You might consider the Antec Sonata II case with 450W PSU, that should work well enough. You probably won't be able to upgrade much with it since 450W is skimpy, but it would run what you have right now just fine.
Might consider Samsung's 18X DVD-burner, if he really cares.
AB9 pro might be a better mobo, but either way, all the top tier 965 mobo's are similar. DS3, P5B, AB9, find which ones cheaper and fits you better.
I really would get the 320 gig Seagate PMR drive. If not, Fry's is selling a 400gig IDE drive (yes, IDE) for 90 bucks, which is a damn good deal. If he cares more about storage than speed...
I agree on the memory, get DDR2 800 at the most. If not Ocing', 533 is fine.
Screw the Zalman, overpriced for what it does. Get an Arctic Cooler Freezer 7 pro.