kinda late i know but information to anyone needing it. i have the same cpu paired with a hd 6790. with my old ecs am2+ board it was bottlenecked (didn't see much improvement from my 9800gt). after i swapped my asus M4N68T-M V2 the video card picked up a bit. i am assuming it was the swap from pci-e 1.0 to pci-e 2.0 that helped. i run it with a cooler master gx450w with no problems. haven't OC yet so don't know if that will help out too, but i may not bother as i am getting a pII soon
Have a look for a phenom ii x3/x4 or try unlocking your Athlon ii into a 3/4 core. Can you unlock the cache as well with the cores? I think you will be slightly bottlenecked with a dual core CPU. What wattage is the Corsair CX500 PSU? If it's 500watts then its' fine.