Nvidia, for the simple reason that you dont have any problems updateing your drivers. I had the 8800GTX. It's a very nice card. I own a 9800GTX now and there is not much dif between the two. I'm in the process to upgrade the heatsink on the gpu. It's a bit noisy when running at high rpm, but I have no problem when I have my earphones on.
Depends on the PC itts diferent With ATI or Intel processors... Where the intel chipset would fit better with the Nvidia and ATI with the ATI.. its easy just buy one that suits your PC they both have their good side