1)7600GS and 7300GT GDDR3 are better than 6600GT despite whatever you may say. 3D Mark is not very accurate with real time benchmarking scores.
2)
AFAIK A new 6600GT should cost around 5500 or so. So, your card should come for around 4500-5000 on the higher side if you offer 1.5-2yrs or mroe warranty.
@buyers,
his card is box overclocked or overclocked