I vote for the 4870, when you enable very high AA and AF settings the 260 can't compete. The AA penalty is much lower on the 4000 series.
However if you don't care about uber-settings, the GTX 260 is far from being a bad choice. If you 'like' nVidia you can safely get the GTX 260, performance is similar.
Originally Posted by nvToni
I would go with ATI but my main conserne is only 512 Mb of memory and only 256 - bit bus
I play at 1920x1200 and was scared about this too (previous card was 8800GTX). However no matter what game and settings I throw at my 4870, it just... flies. Crysis is the only game where the 260 has a very slight advantage, however I'm sure it will be fixed in future driver releases, because the 4870 is faster in every other game.
BTW don't believe the idiots who say that 4870 is crap or ATI drivers are crap, those times were gone long ago, but some people prefer to live in the past, or they are blinded fanboys, avoid them like the plague