XFX 7900GT Framerate Issues, please help!

http://www.newegg.com/Product/Product.asp?Item=N82E16813130037

I got this MSI motherboard for the computer that I'm using right now. It it the first computer that I've built by myself and I'm pretty sure that I "done found me a good'n" when I chose this mobo other other mobos that were PCIExpress x16. I'm not really good at troubleshooting my problems concerning this and other products, but maybe someone here can help me finish up becuase I'm sure I'm doing something wrong.

http://www.newegg.com/Product/Product.asp?Item=N82E16814150139

This is the videocard I put in my new rig, which works. But compared to other people with similar systems, my benchmark scores really suck. I always get <60 FPS in Counter Strike: Source stress test (oddly, 110 FPS if I'm running it in windowed mode) and a score of about 3400 points on 3dMark06. I'm sure that something's wrong. I sent back my first card, which had the same scores as the above, but it was artifacting really bad. I got my replacement card in today, and was dissapointed when I got repeated results. Most other people are getting 100+ FPS in CS:S stress test with highest settings with this card.

My rig:
AMD 64 Athlon 4000+ Single Core San Diego
MSI k8N nFore 4 SLI X16 Diamond Edition
XFX 7900GT Extreme Edition
2GB Corsair XMS
Western Digital 74GB 10000RPM
ASPIRE 520W Power Supply
Xion II Case
Logitech X530 5.1 speakers
Creative X-Fi XtremeMusic soundcard
Windows XP Home w/ SP2

Other people have helped me with checking to make sure the RAM's voltage and timings are alright, making sure that the 6 pin power cable is plugged in, getting my video card and nForce 4 drivers updated, disabling the X-fi soundcard and trying it with onboard audio, and some noted that it may be heat. From the reviews on newegg.com, I already know that this card gets hot. I was getting 75 degrees celcius on Counter Strike: Source. I was thinking about getting a better fan, A Zalman VF-900 CU and putting it on there, but still people are getting 100+ FPS on Coutner Strike: Source out of the box. Any suggestions would be greatly appreciated. I'm currently using the version 84.21 drivers. Thanks in advance.