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.
