I just upgraded the rest of my system. The only thing I didn't touch was the PS or Video card. The reason being, I wasn't sure it would be worth it. But if the overall opinion is that the GTX 580 is a worthy enough upgrade from a 5850, then I would do it. A new PS would be necessary as well of course.
CPU: i5 2500K
Motherboard: ASRock P67 EXTREME4 GEN3
Memory: G.SKILL Sniper Series 8GB DDR3 1866
The PS I will upgrade to: SeaSonic X Series X-850 - http://www.newegg.com/Product/Product.aspx?Item=N82E16817151102