The xt has higher core and memory clock speeds, and has some other optimisations, but overall it's about 10% faster than a regular 9800pro. I don't know the details of the optimisatons, but when I flashed my 9800p with the xt bios, my benchmark scores increased, even when I "downcklocked" it to regular 9800p speeds. But it's not the kind of difference that you'll notice when running games.
If you can get the xt for $100, then go for it, it's a steal whichever way you look at it. But paying retail price isn't worth it for an xt. Anyways, I bought a zalman cooler for my, and it now runs 455/370. The stock 9800pro speeds are 380/340, I think. I'm seeing about a 15-20% improvement over stock settings.