NV Silencer 5 Not Cooling 6800GToc. Need Help.

I'm new to the forums in hope of finding a way to fix a small problem.
Here are my current system specs:

AMD Athlon FX-55
2GB DDR400 Corsair XMS
Gigabyte GA-K8N Ultra-SLI
BFG Tech 6800GToc

Nothing is overclocked.
Using the stock cooling on the graphic card it idles at 64-68 degrees and when I play any type of demanding game it will overheat after a good 5 minutes.

I purchased the Arctic Cooling NV Silencer 5 (rev.2) after reading alot of reviews about it. I also purchased the AC Thermal Compound for this.
I installed the new cooling unit this morning and I am still getting the same problem. Idling at 64 and after 5 minutes into HL2 it overheats and causes the system to crash.

I read mutliple instructions from multiple sites and it seemed quite simple on how to install this, and I did all this with ease. So what could possibly cause the problem here?

Any and all information would be greatly appreciated. Thanks in advance.