If it's just a vanilla X800 then the X1600XT will be quicker.
With respect to current generation cards, the X1600XT falls between a 6600GT and 6800GS. Previous generation cards are also viable options, but they lack more advanced shading, lighting, aliasing and filtering support. From a performance perspective, the X800GTO will run with an X1600XT and an X800GTO2 will be out in front...
At the end of the day, the X1600XT is a decent and capable card. From what i've seen, pricing in the US is a little off tho... it's priced so close to a 6800GS that you might as buy a 6800GS as it performs better.