or wait for the 2900PRO, which is cheaper than a GTS, has more video memory, performs better in benchmarks AND costs less..
if you don't feel like sparing the extra cash, get an X1950PRO for sure. If you really want super cheap, get an X1950GT (X1950PRO with lowered core speed) and overclock it to PRO speeds. 7900GS are more expensive than the X1950s for some reason and the performance difference is negligible. X1950 is the true value king.
if you're gonna go mid-range DX10, as said before, nothing lower than an 8600GTS
. Also, I've seen some reviews for the X2600XT and it seems to be between an 8600GT and the 8600GTS, and it's pretty dirt cheap. Runs a wee bit slower than an X1950GT with DX10 capabilities for less than or about the same price.