I disagree. The GTX580 is more powerful then the 5870, and at 1920x1200 with the most demanding games you'll need that power. Assuming you have the $$$ it will be well spent. This is assuming of course you play any of those demanding games. If you are happy with the performance you have, then don't bother.
Yes, that's why I said 'graphically intensive' games.If you play CoD or WoW, you wouldn't notice any difference.