FPS and Performance problems

Alright first my specs:
Manufacturer:
Gigabyte Technology Co., Ltd.
Processor:
AMD Athlon(tm) 64 X2 Dual Core Processor 6000+ (2 CPUs), ~3.1GHz
Memory:
4GB RAM OCZ Sli
Hard Drive:
713 GB Western Digital Greenpower(1 TB)
Video Card:
ATI Radeon HD 4850 Series
Operating System:
Windows Vista™ Business (6.0, Build 6001) Service Pack 1 (6001.vistasp1_gdr.090302-1506)
Motherboard:
GA-MA78GM-US2H

Now with regards to other parts, I've swapped and tested the CPU(changed it to a Phenom 2 X3 710), mobo, GFX card, ram, am testing the HDD right now and everything seems to be fine. I'm wondering if my OCZ ModxStream 500w could be the culprit of the performance loss. I've reformatted with XP, Vista 32bit and Windows 7 64bit, updated drivers and every other obvious thing I should do.

The low FPS problems would be getting lower then 30fps when running Crysis and Farcry 2 low, Battlefield 2142 around 30-40 fps, Neverwinter Nights 2 under 30 fps on low, under 30fps for Fallout medium.....you get the picture.

No temps are fine for the system. My harddrive testing has come back with 0 errors, now the only piece of hardware that has not changed is the PSU.

No, which is my real problem, really messed up issue. I remember having the same problem years ago with an AMD XP build, but I can't remember the solution if any.