When I build my new PC I'll stick with the 9600GT, the 9800GX2 seems a bit overrated for the price and doesn't seem to offer much more in terms of performance over the 8800GTX/Ultra.
Nvidia hyped this up and made it seem as if we were going to see a mega increase of FPS and performance in games such as Crysis when the gain in performance is quite negligible and quite meh compared to it's predecessors.
Right now it seems the best bang for your buck is either the 8800GT/9600GT and possibly the 8800GTS.