There's not enough performance difference to really seperate them for games although the HD4870 has better HD playback if that is importaint.
Get the GTX260: It has a better warranty and BFG offer a 'step up' programme if you live in the US or Canada...GRRRRRR! I'm in the UK!
Well, the cooler on that 4870 will keep it cooler and help you to overclock it. The extra RAM on the 4870 will also become more important in the future as games take up more and more RAM. If you play Crysis alot though then you would appreciate the 260 sp 216 a little more ^_^