A 400 watt will handle it fine but it's definitely not just about the wattage. What's more important is the amps on the 12 volt rail of the power supply. It should have sufficient amps to power the graphics card and all the other componants in the system with extra to spare. Also, a quality power supply is super important. It may cost a bit more but there is no single component more critical to a smooth and in particular, stable running PC than a good power supply.
I have an old 6800 GT that I used years ago in perfect working condition. It too only has 128MB RAM but it's faster than your 6600 GT. PM me if you're interested and maybe we can work out a deal.