Getting really low performance, need help

I know this is more of a hardware question, but since I didn't see a general performance board and this is a gaming rig, I thought maybe you guys could help me here. Here's the specs:

ASUS M2N-SLI Deluxe Mobo
AMD Athlon 64 x2 5400+ Black Edition 2.6 Ghz
NVIDIA Geforce 8800 GT x2
4 gigs of RAM
Windows XP Home

Naturally, my expectations are pretty high for the machine. However, I get pretty dramatic framerate drops playing games like Far Cry 2 and Fallout 3 with max settings, going as low as the 20's. My average temps are about.

GPU: ~50 C idle, ~65 C load (fan speed is all the way up).
CPU: ~40 C idle, ~55-60 C load.

I thought maybe overheating was the problem, but after searching the net a bit these temperatures seem to be pretty standard for the hardware. However, I am open to any suggestions and all, and all advice is greatly appreciated. Thanks a bunch.

What resolution are you running? Unless very high res, the stock clocked X2 5600+ looks like the weak link vs a pair of 8800GT's.

I don't think it's a temp issue. CPU is on the high side for stock clocks, but not terrible and within range; GPU is totally fine. What cooler are you running with that Black Edition? If it were a non BE I'd assume a retail AMD cooler.