Hmmm... so actually the GTX 560 is closer to the 6870 and is the best value of those cards:
What PSU do you have (brand and model, not just wattage)? The HD7770 sips power compared to the others, but it is also the weakest one. Assuming your PSU can handle it, of those I'd try to get the HD6870. It is the most expensive one, but should allow higher settings than either of the others.
Good charts, but it may depend on the games. OP, check the specific [types of] games you play to see if they strongly favor nVidia or ATI.