X1600 PRO will perform 80 to 90% of an X800 GTO in most things, although there are a few games that the X1600 architecture REALLY HATES (like COD2 I believe), where it performs dismally.
Then again, it has better DVD playback/encoding, and more advanced shaders for upcoming games. So it's not a horrible choice, but I'd also give the 6600 GT a serious look. It's more consistant.