Games running slower after upgrade?

Hello,

I'm in a bit of trouble, I recently upgraded to an AMD Athlon 64 X2 6000+ 3.1ghz, 3gb DDR2 ram @ 667mhz and a Gigabyte GA-M61PME-S2 motherboard. (I still kept my graphics card, 8600gt). I used to have some Foxconn Motherboard, Pentium D 2.8ghz and 1.5gb ram @ 667mhz.

I used to play games like Splinter Cell: Chaos Theory and Battlefield 2 on my old pc, both of which ran very well on high settings.
I then tried them on my new rig and it wasn't good, it seems they run worse than they did on my older computer on the same/lower settings.

I've reformatted my whole PC, and still no improvement, defragmented my Hard Drive, all that stuff but nothing's happened...

I've got 2 theories, one is that my graphics card is just getting old and under-performing, and/or getting too hot, my other is that my PSU isn't powerful enough. I've heard that an insufficient PSU can cause crashes and total hardware failures but not under-performance from components.
The PSU i was and STILL AM using is 300w, which i know is scraping off the bare minimum.

Any help would be greatly appreciated  

Thanks,

Airsoftspock.