As stated: it depends on your situation. If you need a more powerful card right now because of a current game you're really into, go for it.. however: I wouldn't recommend the 6xxx series; yes it has a lot better power management and a little better intake, but it is slightly less powerful. If you can upgrade to a 5870, or even a 5970, with your current PSU; I'd say it depends on the money factor which you get.. just because the 6xxx series is newer doesn't make it better, the best graphics card out at ATI atm is the HD 5970 imo.. however, because of ATI's poor driver support I will be a nVidia user in the future: I have my 8800GT @ 512MB superclocked for over 6 years before finally upgrading to a HD 5870, and I regret it. Sure, it's a lot (so much) more powerful but the drivers suck.. I've had to reinstall / install the drivers about 11 times now just to find one that works.
It's really not that they "bother me", it more of the fact that they're so unstable, you're almost scared to upgrade. Like, with my 5870 I upgraded to 11.2 today from 11.1a. I saw a -40FPS decrease in an area I've been playing around the most in my game where I constantly got 100-120FPS. As for the more powerful DX11 engine: by the time DX11 is required for gameplay, it'll be time for a newer graphics card.
there aly more than a dozen full DX11 games out and many more are coming out this year. Nearly a year and a half ago, when the the 5 series was out, DX11 was irrelevant. Today, you want a good DX11 card, unless you plan on swapping cards in a year.
Again both the 6870 and 560Ti are very good for thier money, and are not gigantic power sucking cards that can break an average PSU's back.