The X1600XT is certainly better at stock speeds, but if you can unlock the 6800XT you've got a great card. Of course, the 6800XT is also $31 cheaper, so you'd expect more out of the X1600XT.
The X1600XT is certainly one of the better cards under $150, but if you're willing to stretch the budget a little, the 6800GS is a great choice.