Problem with MSI R4890 using Asus P5KC

Hi

Yesterday I got my R4890 to replace my older MSI 8800GT oc. Running Steam games like Team Fortress 2 I noticed framerate drops now. I use a dual core asus P5KC motherboard with a E6750 proc. I placed the card in a friend's quad core computer and all is ok with outstanding graphics. After that we placed the 700 watt psu + 4 gig int. memory in my computer but still the same problems. Also a new Vista 64 install did not do the trick. In same cases the 8800 ran even better. According to the manufacturer my minimal specs to run the 4890 are sufficient. Do anyone of you have ideas or suggestions?

Thanks

Michel

My setup
P5KC with E6750
2 gig ram DDR3
MSI R4890
500 watt psu
Vista 64