I'd go with the 5870. It's cheaper than the GTX480 and supports 3 monitor Eyefinity. I have an Eyefinity setup (3x 1920x1080 21.5" LCD's) and it is great for gaming. Most games I don't really notice the (fairly thick) bezels on my monitors.
If you want Eyefinity I'd definitely say 5870 minimum, maybe I'd look into the 5970 (it's a single card that has 2 5870 GPU's slightly underclocked) but it is expensive and also eats power. The 5870 is very decent with power consumption, it uses at most 175W and has great idle power levels. The 5870 will play Crysis at full settings on a 1920x1080 monitor, but lags pretty bad with 3 of them. It's not a problem if you turn down some settings to Medium, then it plays smoothly across 3 monitors. My main game is Team Fortress 2 though, it runs fine at full settings (except AA) on 3 monitors.