I've got the exact hardware you're proposing. My motherboard is the Dual VSTA (basically an earlier revision of your 4CoreDual VSTA) which also has a x4 pci-e slot. Before I took the plunge on the x1950xt, I read the following article:
http://www.tomshardware.com/20...ress_scaling_analysis/
Normally, I don't read Tom's Hardware. However, this article is pretty good. They tested performance scaling on an x1900xtx and an 8800GTS across x1, x4, x8, and x16 pci express speeds. To make a long story short, the x1900xtx didn't take too much of a hit even at x4 link, while the 8800GTS suffered pretty badly at anything but x16.
So, if you're thinking of getting an x1950xt, go for it! You'll take a slight performance hit from the x4 pci-e slot, but it'll still be a lot faster than your 6800. Hell, I came from a 6800GT (a bit faster than your 6800) and I noticed a huge performance improvement.
Cheers,
Dave