Personally, i wouldn't go XT. The 1950Pro is a fine card, and very cheap. The XT is faster, but not enough to justify the extra $$, espeically with the recent 1950Pro price cuts.
149.99 after rebate For a x1950Pro is a great deal.
Also the nVidia 7900gs is priced similarly. Both perform very close, so it's a choice of preference between the two.
Generally hardware is priced along a curve that gets very steep after you pass a certain point. While the XT is faster, you'll have to decide if it's 'enough faster'. Personally if I were prepared to spend the money for an XT, I'd wait until I could get a next generation card like the 8800 or the R600.
This is a very handy guide to help you compare your options.