If you are going to spend $500-600 buy a radeon 7970.
If you are gaming at higher than 1920x1080 then more than 1gb of vram is suggested. Some people even report maxing out their 1gb vram (with ultra-res texture packs) even at 1920x1080. Generally speaking though, 1gb is enough vram for 1920x1080 and lower.
I haven't looked at specific prices but a gtx570 will do 1920x1080 really, really well. A 560ti is enough for that resolution, but a 570 just does it better and is much more affordable than a 580. All of these cards SLI/Xfire very well for future upgrading if your rig can do it.
Waiting for Kepler depends on how badly you need to upgrade now. I wouldn't buy a 580 - since the 7970 is out and comparably priced. A 560ti, 570 or 6870, 6950, 6970 are all very nice and are magically priced close to their performance ranks. Personally, if I could afford it, I'd buy a 7970, but I game at higher than 1920x1080 - your situation may be different.