I agree with Doc that both of those cards are great. I'm actually using the HD 6950 and haven't found a game I couldn't comfortably max out yet.
The only thing though, is the 660 and 7850 are both out of your price range, even more so if you need to upgrade your PSU.
Can you post your specs, and tell me what PSU you're currently using?
I'm thinking the 6850 would be well within your budget and it's a pretty solid card for the price.
and even if you gotta upgrade the PSU you can find decent 500-600 watt ones pretty cheap.
Gotta also keep in mind space.If you're using a mid ATX case the other two cards may be a bit too snug,
I would also be concerned with core temp. as well.