Re: Graphics cards
I have the VisionTek 5670 and I haven't done my research on it so this post will be mostly anecdotal.
Basically, the graphics performance vs. price is great for the 5670 (although the version I have is 1GB vs. the 2GB you linked). The compatibility though, is less than great. Linux has a lot of trouble with the 5670 and many of the distributions I tried simply wouldn't work with the 5670. Not sure if this has been corrected in the later cards (6850) but I imagine that from the reduced number of Google results for a 6850 Linux search, and the fact that most of the posts have an update to them with a note about updated drivers, your experience will be a little better.
Assuming there's no CPU bottleneck, I would recommend (and have read) that the 6850 would be much better. (Especially if you CrossFire those later) There's some discussion floating around that the 5770 = 6850, but this is not true. There are a few noticeable differences especially when AA is enabled. I'd personally go with a 6850 out of those two options and then CrossFire later when additional funds become available.
Humans are the only creatures that won't live up to the their potential. Give everything your all. When you die, die on E.
So I ask you, what do you intend to do now?