Think what you want, Hacp. I would happily buy a 7800 card if Nvidia would just clean up their image quality problems. I'm not gonna plop down 400 buciks on a video card only to get in game and see the ground textures wiggling around and waving at me. If I want to see a psychedelic pixel display, I'll turn on WMP and watch the visualizations.
You are so full of BS, it isn't even funny.
As for the OP's question. With your system a 7800GT would be the best card out there. The GTX will be bottlenecked by your CPU. As for which card is faster, the GT and GTX are much faster (Once you crank the resolution and detail settings). The GT and GTX frequently match or exceed SLI 6800U's.
Image Quality, they are right on par with each other, with the nod in filtering going to ATI. However, you can increase this more (with a speed loss) by enabling Transparent AA.
The X800XT is a good value if you can find it for <~$320. Any more than that you might as well go with the GT. Additionally, if you plan on keeping this card for any great length of time, (Maybe skipping this generation), you will want a 6 Series, or 7 Series card because of 32bit Floating Point Precision (allows support for OpenEXR HDR), as well as SM3.
There was an ATI deal on the X800XT on ATI's website, but that has to be back ordered for MONTHS. Im not even sure if they are continuing to sell it. Unless you get that deal i would recommend a 6800GT.
Having said all that, IF you can possibly wait another 1 or 2 rumor has it that ATI will be releasing their next gen midrange cards, which look very promising by the specs. You could wait until all the next gen cards are out and decide then, your choice.
-Kevin