In newegg, the cheapest X850XT PE is 385 USD.
The cheapest 7800GS is $299 while OCed GSs are $319.
Now the GS can be oced around 480~/1400~. It is S.M 3.0. Power usuage is about the same, but the GS is single slot, and produces less heat. Firstly, the GS offers you TRAA (hence you can use either TR MSAA, or TR SSAA). It also comes with pure video if you want high quality DVD playback etc. When the GS is oced, it beats the XT PE by about 5~10%, even more in OpenGL titles. HDR is also a nifty feature you get.
I would choose the GS, as the X850XT PE wont be able to play S.M 3.0 titles. These cards of now should be using 10x7 or 12x10 resolution.
Another option is option is to move to PCI-e. The cheapest GT is at $270, a EVGA model. im sure, getting a s939 mobo and the GT is far more better than getting either the GS or XT PE.
edit - the 7800GS can run S.M 3.0 games fine and another thing to note is that IQ will look better due to GS having P.S 3.0 unlike the XT PE shown in various games like AOE III. You have to remember that these cards perform very well at 10x7 or 12x10 resolutions. Higher resolutions are for the higher end cards like the X1800XT or the 7800GTX.