1. 800mhz RAM is faster for certain applications like video encodes, but usually you won't see too much difference. I'd still say get 800mhz as it isn't really any more expensive.
2. 8800GT by a small margin, but a similarly priced HD4850 will beat them both handily.
3. No.
1.) DDR2 800 actualy runns at 400MHZ per stick...a pair of them will run together at 800mhz. That said, two sticks of DDR2-800 is more than twice as fast as one stick of DDR2-667. If you are going to spend the money for a 8800gt/9600gt/HD4850, then do yourself a favor and get four sticks of memory, that way you have a pair of 800mhz banks to play with.
Sorry, what? Two sticks of memory run far better than four. If you need the extra space, double the module size. Having used 4 slots, it doesn't degrade performance, but 2 slots works out better, especially when overclocking.
Using a Phenom, I get better performance from 4 1GB sticks than from 2 GB sticks, but that might be because I could not find 4-4-4-12 2GB sticks when I bought. As for the 4GB sticks...I still can't find any CAS4 models. I think a lot of the performance issues with 4 sticks have more to do with the memory controler...there is twice the theoretical performance there if the controler is done right.
