If you have had a 260 this long, surely you can give it a few more weeks . You will be happy you did. April is the expected release date of the first cards, and the lower end (GTX 660) is rumored to give you better performance than the current 580's do right now, at a lower price!
What games are you playing currently and what future games you want to play? DO you have a 60HZ, 85Hz, or 120Hz monitor?
I had a 260GTX and upgraded to a 560Ti 2GB and it was DEFINAtely worth it for me. But I like to play Gran Turismo 3 on PS2 emulator with a 2kx1k+ resolution (instead of the native) and the 260GTX 896MB framebuffer was not cutting it for me there. ALso I have a 120HZ display and I was rarely getting over 100fps in Team Fortress 2. Now I get 120fps easily in that game, it's basically maxed out for my display now I couldn't be happier with its performance.
Your in a bad spot though because the Kepler cards have been leaking info. (So they will come out this year it appears.) I don't know exactly what to tell you based on limited info of what you play. I guess if you don't play any DX11 games at all, and don't see any you like you could try to wait, or try 260GTX SLI for low cost that is if your mobo is capable for a performance boost (if said game(s) enables in SLI). Yet the power consumption would be very wasteful doing SLI with older cards (They downclock to like 300-400mhz whereas newer ones downlock to 50mhz or less) so they don't compared to the Nv 5xx or AMD 7xxx series when it comes to idle power savings.
I think if your a [H]ardcore gamer you probly can't wait 2 generations, so maybe it doesn't matter to you.
This. There will never be a worse time to buy a 580. I can see getting a deal on a 560, or a 6950, or a bunch of other cards that will be an upgrade AND a decent value if you get them on sale. The 580 would have to be pretty darn cheap for it to be a good value at this point though. And the used price on these is going to plummet also.
edit: I went from a gtx 260 to a 6950, and although the performance boost was awesome and worth it, if I could do it over again I would go nVidia. Many people have no driver issues with ATI, I have.