What version drivers are you using for your 6600gt card? I believe that card needs v. 81.98, but you can also try v. 82.14 which are beta drivers from nvidia. I've heard good things about the beta drivers.
Also, what version of direct X are you running? The latest version is directX 9.0c which can be downloaded from microsoft HERE
Worst case scenario is that your PSU can't handle the card. The minimum requirements for the card is a 350W psu, and I believe that nvidia recomends 18 amps on the +12v line, and probably recommends 20 amps. I found out the hard way that my psu couldn't handle my nvidia 6800GS, even though it met the minimum power requirements, it woulnd't power the card. I installed it on my sons machine which has a 500 watt psu and it works fine. Needless to say that I had to order a new psu for my computer. Just something to keep in mind if all else fails, it might be your power supply. But try the other things and we'll hope that they work!
War produces veterans, wounded both physically and mentally. They have sacrificed for us.....and it is now our job to help these veterans, as they have already helped us in ways we will never know, in ways that we cannot fathom, and in ways that we take granted every day.