it's not a "real" upgrade unless you get the 6600gt for really cheap and sell your 9700p for top dollar

(maybe ~ +15% more performance)
i'd suggest waiting till you can afford a 6800gt . . .
what resolution do you game at?
at 10x7 the 9700p will handle ALL modern games at mostly mid-hi details with mostly everything on and max . . . . the 6600gt will perhaps give you everything hi at 11x8 and it is too anemic to display SM 3.0 pathways in current games . . ..
by the time the 9700p needs upgrading, the 6600gt will be getting "slow" also . . . you can look forward to price drops this Summer as the new ATI (and probably nVidia cores) are released.