1. Run Win 7 x64.
2. Depends on the game and display resolution, along with what else you are driving. I'm guessing you are doing multi-monitor 1920x1200 or higher resolution with that much GPU power. Some high resolution games + OS + a web browser or two will eat 4GB of memory fairly quick. Realistically, you want as much cached there as possible.
3. If you had 6GB, I would say you are going to run into a CPU bottleneck well before RAM.