Personally I don't think a GTX 260 to a 5850 is worth it, but to each his own. It varies wildly depending on the game of course, but you're looking at a 30-40% improvement over a Core 216? Maybe if you are running a 192 it is worth it, but I wonder what games you you held back in and at what resolutions?
Personally I feel that the proper "next step up" is minimum about as fast as two of your cards in SLI or Xfire. For GTX 260, that would be the Radeon 5870. If Nvidia aggressively markets the GTX 470 then we could see the 5870 close to $300 sometime this calendar year.