I think the X700 is better due to the 6 vertex shaders and 8 pipes (vs the 6600gt 4vetex 8 pipes)but others will say the gt is better due to ps 3.0 & SLI both are true
then again it been rumord the 6600gt was really a 4X2 4 pipes 2 texture units card kinda the way they lied before with some of the the 5XXX series. But there is no proof other then some bandwith tests that people are going by that are discussed but never seen.
In the end after what ever it is said here it up to you to make an informed decision.
But it depeands in some cases ATI will be better others nvidia.
In this case your probubly better off with the GT for the long run. Or wait and see the new ATI line up as well as the nvidia line up... that will come show up this fall