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.
if you want good bang for that buck, the 570 with a proper OC can perform with a stock 580, and i think that is well worth the money, with better tesselation performance especially going from the 5xxx series radeon. it really depends on what you want to play at which resolution
I play at 1920 x 1080. As far as games go, I play a very wide range. After the recent Steam sales over the past month or two, I've picked up a few "new" games such as Metro 2033 and that is quite the demanding game. I'll be picking up Skyrim. Granted, being a console port, I'm not sure how demanding that will be.
In the end, I like to be able to put the graphics all the way up or nearly all the way up and leave them there. When I have to start turning stuff off to keep decent fps, that's when I start looking at an upgrade.
i run a 570, and have no issues on very high with any game, even metro 2033. There are a few settings that i stay away from such as advanced DoF, but everything else maxed. Witcher 2 runs great as well with ubersampling off around 50-60 fps@1920x1080. If you can wait, the new radeon cards should be coming out in a couple months, and be much faster than current gen. if you are wanting to buy now, then i think the 570 is a good choice; it is much cheaper than the 580, and will perform extremely well at your gaming resolution.