If you can get $150 for your XL, do it. Don't hesitate.
X850XTs are selling for mid $160s AR shipped, and should only get cheaper fast as the 7600GT drops below $200, forcing the 6800GS down from $170. It's not like ATI is going to run out of those cores any time in the forseeable future. And those 128 bit 90nm 7600 should be cheap for NV&partners to make, so they definitely have the option to completely punt ATI out of the low end. I wouldn't be shocked to see 7600GT prices coming down fast & hard to $150-ish in the next month or two.
The gain of a 7900GT over an X800XL with no overclocking is not quite 2 resolutions at the same frame rate. Or 1 resolution + more eye candy. You make the call.