BI has said ArmA was optimized for Nvidia cards. I believe they are working with PC Nvidia equiped.
I am playing ArmA with an ATI card, now an "old" MSI 1950Pro 512, playing fine on high, has to disable Anisotropic and Post-processing options to have a nice game. I was playing things smoothly while some people were having issues with high end Nvidia cards.
Some problems have been reported on ATI cards user since the release of the 4800 serie, and BI has announced that this problem with ATI card has been solved for ArmA and ArmA2, I have read something about that in an
ArmA Database Mission interview [of course it's in German ...] but you can understand that now, ATI card users can use options in the "Video options" in order to correct the problem.
From these bits of informations, I will conclude that it will be easier to play ArmA2 with Nvidia card without having to tweak the video options, ArmA2 will be playable with ATI cards with some works on the adjustments ... with less money needed
I am working on the same question GTX275 or ATI4890 for myself ... and I will wait a bit after the release of the game, reading the forums and asking questions before coming to a decision, for spending more than 200€ on a video card is a difficult move even for a dedicated ArmA player/fan as I am.