Wait until the new 560ti's get upgraded in december, and get 2 of them for sli. That would be what I would do.
What do you mean with the 560 ti upgrades? Like the new Kepler architecture?
Thing is, with technology you always end up waiting and waiting and waiting....look at me... been waiting 5 years with my 8800 gts 512 mb
I wouldn't pay for a 580 right now, they cost a lot for a minimal upgrade over the 570. If you wanna go with something that'll be worth something after the ati 7000 or the nvidia keplers, go with 580's little brother... the 570 forz III.
If you can wait it out... wait for the black friday (they usually have boxing day like prices!) and grab yourself a low cost 580 or an even lower 570.
EDIT: Wow I didn't know that nvidia would modify the 560ti in december :P Well while you're there ... wait for the next generation lol !