At stock, it goes something like x800xl ~= 6800gs > gto2 (unless yours has 16 pipes unlocked already)
When unlocked and/or Oc'd, it goes like this: gto2 > 6800gs > x800xl
The xl is a poor overclocker in general, so for better performance out of the box get the gs, and for better performance when modded get the gto2.