phil113 :
one last thing though, will this be a huge boost from the nvidia 6800 in terms of playing the games on a higher setting?
I have a similiar system. I am currently running the Geforce 9800 GT on my XPS 430 with a 375W PSU and have had no issues. The card is wicked for the price. I mainly play WoW and run the resolution at 1600 x 900 with all the visuals at their highest and my FPS are steadily between 35 - 60.
I also play Crysis (which is beautiful) and a few other 1st Person Shooters and love the graphics!!
You can't go wrong with the 9800 GT, but as ct1615 mentioned, you could also run the GTX 250, which would give you an increase of nearly 30% from the 9800.
Good luck with your choice and "Game On"!!
Chris