Welcome to the forums,
Are you looking to add another card in SLI ? or do you mean as a replacement ? I beleive if you are planning to SLI that the cards have to be the same. IE 8600+8600.
Yes ATI need same generation IE 4850 would go with a 4870 and i guess that includes the 4670 and 4650 now as well
Nvidia needs the same card. There is one differance with Nvidia and im only including it for info as its not relevant to your situation, but the 8800 chips are G92 which includes quite a few cards and im not sure if its the card specifically or the chip that is the issue. Now if its the chip then 8800 owners have the 8800GTS /8800gs and the 9600GSO which is the same card as a 8800GS. Nvidia always did like to keep it simple