As far as advantages, the X800XT has a bit more brute strength, where the 6800 series is almost as powerful but sports more advanced shaders.
The X800 series tends to be better with most DirectX games (HL2, Far Cry, UT) and the 6800 series tends to be better with OpenGL games (Doom3).
The 6800's advanced shaders are mostly useless right now but may come into play more in the future. Right now, the advanced 3.0 shaders the 6800 sports are only used in a hadful of games, and cause no real difference in performance or visual quality except in very specific circumstances (like Far Cry, where it allows you to use High-Dynamic-Range lightin effects).
If this is a temporary upgrade - when are you planning to upgrade your platform, and to what?
I'm getting the impression that you have an AGP system right now. If so, the 6800gt or ultra might be your best temporary upgrade bet, because it'll give you 90% of the performance of the X800XT/6800 Ultra for much less price.
The ASUS 6800GT is expensive because it's got the ASUS name brand. Look around, there should be cheaper 6800GT's than that.
________________
<b>Geforce <font color=red>6800 Ultra</b></font color=red>
<b>AthlonXP <font color=red>3200+</b></font color=red> <i>(Barton 2500+ o/c 400 FSB)</i>
<b>3dMark03: <font color=red>11,506</b>