I'd sell the 5900 Ultra for around $150 used. Yes, the 6800 GT is worth it, all $400-$450. Mine is way too powerful for how much it cost.
The eVGA OC's extremely far as well (mine is at 427/1200 MHz when the default is 350/1000 MHz) with stable results.
If you upgraded you'd notice a huge difference in resolution and image quality settings. Your PC would be godlike with that processor and videocard combo.
-Lasereth