The 6600GT is a lot better then the normal 6600. Seeing as you want it only as a temporary card, Id still go with a 6600gt if you plan on playing relatively new video games. If you're a cheapskate like me though you'd probably buy an even cheaper card though, like a ti 4 series even if you can get one. The 7900 cards probably arn't to far off so just go cheap.
EDIT- doh I wasnt thinking, you have PCIE. Yea just get a 6600gt then.