For AGP costs are higher.
6600GT has been next to impossible to find for me. You can get 7600GS for 6K minimum.
In your budget you should get 7300GT, I guess.
Yeah. Aero works fine. I have the same card and Vista with Aero is running pretty smoothly. I have got the RTM version for some benchmarkings of the card and it seems to do pretty decently.
7600GS is on par with 6600GT in perfomance, power consumption, heat etc.
Get one from a gud brand nd one with a gud heatsink nd a decent fan too. Itll help u to oc it better. Getting an AGP version of 7300GT is difficult but the 7600GS AGP is widely available.