Problem with new 8800gt (please help!!)

Hi,

I recently purchased a Zotac 8800gt AMP edition which game factory overclocked at 700/1200/1700 (i think). I installed it to replace a 3850 which just wasn't cutting it anymore. After installing the card i've found that my performance in a number of games is worse such as UT3, TF2, Crysis. The only game that benefited was quake wars. I have the latest drivers and i've tried multiple sets, i've used driver sweeper and everything checks out and should work. On ut3 with no AA or AF i'm getting an average of 28 fps with max graphics and TF2 gives me about 40fps on average. i've made sure that it's running correctly on the bus and everything checks out. Where is the problem?

Intel c2d e8400 @ 3.6ghz
Zotac 8800gt AMP edition (512mb)
4gb RAM
320 gb hard drive
Windows Vista Home Premium x64
gigabyte-p35-ds3l

please help,
mersonix