Things seem to get juicer with every passing day. This time, as per a field report conducted my Sweclockers, the GeForce GTX 660 Ti launch is indeed slated for this August and with a more confirmed time frame of when it’s going to launch for real this time.
But if you want to go for more and you can find a matching card and a good deal, perhaps you could go the SLI route (lots of performance there => a pair of 560 Ti's can outdo a 580) - especially if future games really give you a performance hit. If you're comfortable tweaking settings (I know you said you aren't a die hard PC gamer), you can also overclock to eek out a bit more.
Personally, I follow the upgrade every couple of generations pattern (sometimes 3 if my card(s) holding up well with current games).
I've got a pair of 560 Ti's SLI'd and probably won't look to upgrade until they are getting long in the tooth - and when my wallet allows (the 700 or 800 series will be my next upgrade).