Umm well I have had no problems with cards from either company..The ATI card and the current drivers
I am using (V10.10) are running all the games I am currently playing, BC2,Fallout 3 New Vegas,Starcraft2,WoW, without issue.
However I know ATI drivers can be flaky supposedly but I have had no problems with the last 3 drivers released by them. Crossfire tho is a whole nother issue..lotsa problems and I personally would'nt run 2 cards in xfire.
If you are still going with NV I would think that it would be a smart move to wait and see what NV do re a 560..its going to be cooler and will (if the 580 is anything to go by) be a 15 to 20% improvement in performance over the existing 460.