So my choice has come to these two.

Even though 6950 is a card from different category, still it outperforms gtx 570 in some games on windows Furthermore 6950's bios can be reflashed to the one of 6970 unlocking additional shaders and setting clocks higher so it will perform exactly the same as 6970 which is in the same price range as gtx 570 and considered as its closest amd alternative. That would allow me to save plenty of money.

Unfortunately i didn't find linux benchmark for gtx 570, so I can't really compare linux performance.

Also i never ever owned an amd/ati card in the past. But heard some horror stories from people with top ati cards having fps problems in old opengl games on linux and windows before O.o

I wonder if 6950(6970) performs as well compared to gtx 570 in opengl games on linux as it performs in directx games on windows. Also if i can get custom resolutions/refresh modes that are not officially supported by monitor to work with amd card and if there's any alternative to nvidia's vdpau. Using open source drivers is not high on my priority list.

Thanks in advance.