the X800's aren't much behind the 6800's, there is actually very little difference in image quality for most games between shader 2.0 and 3.0
in benchmarks there is very little difference; the 6800 performs better in some tests, and the X800 in others
I think the X800 is a better card because
* it looks better
*it only takes up the one slot
* it uses less power
* it runs cooler, so it doesn't need the big "vacum cleaner" like the 6800 has
* they are less expensive
if you get a chance, try and put in an X800