GTX280 would beat 8800GT sli setup or GTX260 would little short of beating the sli 8800gt's unless you want a ati card. then a 4870 would be close to the 8800gt sli setup but shy of some
I had an 8800GT Sli setup but like what others said here, it suffers from games that do not support sli. I played on WoW for instance, and it had about a 200FPS but sometimes after going through a room of mobs, it'll hit performance hits all the way down to 40-30 fps. I discovered that WoW doesnt like sli and running a single 8800GT would give me a constant 60fps.
I loved the set up until I couldnt stand the noise and heat. I sold em and got a 9800gx2 and I'm happy (still pretty hot, but not noisy).
From what I've seen, I would get the gtx 260 even if it means suffering a few fps. It'll go down in prices just like the 8800GT did a few months back.