You said 640MB 8800GTS which is what I just had. The speed and memory bandwith of that original 8800 is nothing like the newer ones (night and day really). (E6600 stock vs Q6600 @ 3.2Ghz), and you can see what going from an 8800GTS to a GTX 260 did for me:
SLI: I have no idea but it would seem like an E6600 wouldn't even come close to supplying what two 260's or two 4870's could take. Maybe I'm wrong but it seems like a waste. Also a 2nd $250-400 card just to try and max out one game (and all the problems that can come with SLI). It just isn't worth it. Not to mention the PSU needs and the amount of heat generated. I'm very happy with a single card. Be happy with every other game maxxed and Crysis on High at 50-60 fps (1680x1050). SLI is just so much extra money for so little return. The only point where it seems to be worth it is if you need the extra power for an LCD that's bigger than 22".
ATI: I went to the dark side once and had a lot of problems. What sticks out the most in my mind is horrible driver issues with older games. I was tempted to buy a 4870 instead of a 260 but I just didn't want to risk that kind of frustration again.