Sort of....things are a bit complicated for ATi owners, as usual NO CROSSFIRE! Oh and 1GB cards are supported pretty well but the 512MB dudes are having problems and can only play the game at low details...
I believe the textures take up too much memory as my card uses 93% of it's VRAM at times and 89% in typical circumstances...perhaps they do not use texture compression?
ATi users still have the odd z-flicker and texture problems but the game is playable (according to users on gtaforums), however as I said above only the 1GB VRAM cards will have any joy.
4870X2 is a very good graphics card but sadly the developers do not support either Crossfire or SLi, it would be nice if ATi released a driver to somehow sort this for you guys.....but I think it will require a patch maybe?
John