What is best, 2GB or 1GB Radeon 6950, or 1GB 6950 or 6870?
Are they compatible with my motherboard?
Will I need to up my PSU?
I just can't resist to add a bit to this:
The 6870 is a good card, and the difference to a 6950 (with
1GB) is very slim. As long as you don't intend to use a higher resolution than 1920x1080, i'd go for the 6870 if you have to choose between those two. But in the long run the 6950 with
2GB will be the better choice as games will get more demanding and will use more of the memory on your card.
Right now i got two 6870 and so far i've found no game where those weren't enough. My extremely modded (including http://www.newvegasnexus.com/downloads/file.php?id=43135

) FO:NV runs fine, Rage looked (disregarding the texturebug) very nice, and the framerates are no problem as well. The only problem i had with the texturepacks was that i had to take the pack with the medium textures (4.6 GB), as the pack with high textures (7 GB) had a drastic impact on the framerates. There the 1 GB on my videocards didn't seem to be enough... <_<
There's been said enough regarding your mainboard. :goodjob:
Now some things regarding the PSU. 500w should be enough, if you have a good PSU. The problem is that this number on the sticker will not guarantee that it will work as promised. Two days ago i installed a 5870 in the system of a friend and his 650w PSU (something with flower or so) couldn't handle the card. With an old 500w Techsolo (and that one is definitely not a good PSU) we got his system running again. The system is a bit shaky, but he can still do something 'til his new PSU from Corsair arrives.
To make it short: don't believe the high numbers on the sticker of a PSU which doesn't come from a brand like Corsair, Seasonic, Antec, Enermax, or Silverstone. Quality costs, and i wouldn't try to save some bucks on a PSU which could kill way more expensive parts in my system if it dies.