Go for the 6800 GT. Its probably the best bang for the buck AGP card out there now. Plus, it lays waste to even the X1600 from what I see in benchmarks.
Yeah, the X850XTPE sucks. Its $600 and only performs slightly better than an overclocked 6800GT in DX8, it performs worse than the 6800 GT in shader intensive DX9 games and OpenGL.
I hate all these people that say the GT is just as good as the ultra because it can overclock to an ultra. Did you ever think that an ultra can overclock too? It would be like an ultra ultra. So a GT OCd = Ultra, and an Ultra OCd = Ultra Ultra. It is not a waste of money.
Well considering that the 6800Ultra is just a 6800GT overclocked, you can't really squeeze much more out of the ultra. 6800GT oc's to higher than 6800Ultra speeds, and the 6800Ultra oc's to the same as a 6800GT oc's, except for the fact that the ultra costs alot more.