I really feel stupid for asking this, but i think 4gb will have better performance over the 8gb. With the G.SKILL 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 1066 (PC2 8500) Dual Channel Kit Desktop Memory Model F2-8500CL5D-4GBPK - Retail have better performance than G.SKILL 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory - Retail (xxx2!!)
It depends on what you are going to be doing, your mobo, cpu, etc. If you are doing serious overclocking, then the 8500 would be better. If not, the difference is not that great, though there will be some serious speed freaks that will say the extra few, and I do mean few, percentage points of performance is worth it. On a straight performance difference speed ways, they are about the same. Since you are going with V 64, it will use the additional memory, so if you do work the is memory intensive, the 8 gig would be the way to go. I have 8 gig in my main rig, running 64 on a Q6600. I will say the only time I see the additional memory get used is when I am working with my 3D and design apps.
I do some business work that eats up memory, so 8 gig has its use for me. If only running games, than 4 gig is fine for the moment. That may well change in a couple years. For Example, when XP came out 256mg was standard and 512mb was only for those who were really into running multiple apts. Now 2 gig is standard for XP and some people are running 3 or 4 gig.
As to the memory itself, be aware that often times the DDR2 1066 ram is really only DDR2 800 that has been overclocked and uses a bit more voltage. In other words, DDR2 800 runs at 1.8v while DDR2 1066 runs at 2.1v, so all you really need to do is the overclock the DDR2 800 and increase the voltage to 2.1v and viola, you have DDR2 1066. That's exactly what I did with my OCZ DDR2 800 ram.