A 6600 (non-GT) will beat a 7300GS, but is matched by a 7300GT. Essentially:
7300GT ~ 6600 > 7300GS
I'm basing this comparison on the cards' pipelines, texture units, memory bus width and core/memory clocks.
With the exact cards you posted, I'm inclining towards the 6600, but I'm not really convinced. You could also look for a discount (open-box for instance) 6600GT, that's a bit better than both the 7300GT and the 6600.
Edit: also, if you're looking the ATI way, roughly equivalent cards are the X1300Pro, X1300XT and X1600Pro. Both the X1300XT and X1600Pro have 12 pixel pipelines, so they're probably faster than a 6600GT.
So in the great scheme of things it would look like:
X1600Pro ~ X1300XT > 6600GT > X1300 > 7300GT ~ 6600 > 7300GS
All based on reference values. This is a generalization; ultimately it will depend on the particular cards specs, since many manufacturers sell cards with better settings than the reference card. People are more than welcome to refute me.