My only advice is to avoid cards with less than 6GB of RAM, and shoot for the 8GB-12GB range, as these incoming consoles will have 4GB-6GB of RAM to play with on their own, and much of that will be free for graphics; if you extrapolate that up to PC-release levels of detail, you can expect 6GB to be the new 2GB in the near future, with 6GB likely not being enough for the highest detail levels in the top-end games (and I'm not talking AA, but actual pre-rendered game assets).
Consoles are much more likely to be utilizing 2-3 GB for graphics than 4-6 GB. The Xbone OS uses 3 GB alone and run of the mill pc games use 2 GB of RAM these days leaving 2-3 GB for texture buffer. This rumor of consoles having 6 GB texture buffer needs to stop.
Personally I'd wait to see what the 760 Ti and AMD's 8 series looks like. The 570 isn't all that decrepit yet.
Current consoles have 512MB of memory- total. And yet, the compiled-for-PC versions of those games- Crysis 3, BF3, etc.- can challenge the most advanced PC's today. Why? Because developers have been building significantly more assets into their games, and including those assets into the PC versions, as the current consoles are so hilariously decrepit in comparison.
Expect developers to use every megabyte of memory they're given, and then demand more, especially for graphics, which is the easiest thing to scale up. And expect them not to stop at the 5GB or 6GB limit that the consoles give them for the PC releases.
It'd be silly to expect otherwise. Yes, getting extra memory on a graphics card today is hard- but that's a marketing issue, not a technical issue, and it can change overnight. We could have 8GB HD7870's and GTX770's, and 12GB HD7970's and GTX780's tomorrow, if AMD and Nvidia just decided to ship them. The Quadro/Tesla and FirePro versions of these cards already have the memory, it's literally just a matter of a BIOS tweak and a slight shift in the production lines to use denser memory packages.
And it won't even be that expensive- you think an extra 32GB of flash is worth $100 more for smartphones? Flash is dirt cheap; DRAM isn't much more expensive. It's why these new consoles have 8GB in the first place!