Both cards are the same, XL is just newer and uses PCI-E the XT however has an overclocked core, by default it's 100mhz higher giving it 1600MTexels/s more than the XL version. (8000MTexels/s > fill rate)
Like everyone else said (most of them) XT is my choice :thumbup: (higher clock rates)
Depends on your mobo, AGP/PCI-E, or your founds
*EDIT*
Consider the 6600GT /6800 stock, for the price range.