Demios dude, I presume you live in the US. Therefore, heed my advice and get a kick ass "bank for the buck" upgrade from the most trusted, cheap and preferred online hardware shop in the US: Newegg!

Dude, as long as your PSU is about 450-500watts at the very least, here is the DEFINITIVE setup you are looking for....trust me(AMD/ATi ftw). If your PSU doesn't have a PCI-Express 16 power connector, make sure you have at least 2 available molex connectors (the 4 pin connectors that connect to hard- and optical drives among other things).
This setup will cost you a tad bid more than 300$, but nothing you cant handle, dude.

Motherboard: GIGABYTE GA-MA770T-UD3P http://www.newegg.com/Product/Product.a ... 6813128392 = 80 $

CPU: Phenom II X2 550 http://www.newegg.com/Product/Product.a ... 6819103680 = 115 $

And dude...forget about your 8600 GT, it's crap compared to this CHEAP powerhouse of a GPU, the ATi HD 4770 http://www.newegg.com/Product/Product.a ... 6814141098 = 110 $

RAM: DDR3 2 x 1GB Crucial http://www.newegg.com/Product/Product.a ... 6820148194 = 28 $

Shipping costs havent been accounted for, but I cant imagine it being too much.
There you go, so far it is 333 $ and you seriously cant get better performance for that money. Go for it man.

Peace out

Zahid