GTX 480 if you can handle the temperatures and voltages. you will need an aftermarket cpu fan if you want to get any overclocking done with this, but this is hands down the best performance to price contender.
Sapphire 6950 OC 250-50$=200 with code EMCYTZT1834 expires on midnight on July 1st/2nd border. also comes with BF3 code
HD 7850 ~250-260$
GTX 570 ~260-280$
these three cards generally perform about the same level give or take a few fps. the more expensive cards have the slight lead, the 7850 gets special mention as its the only card in this range that only uses 1 6 power pin(most models), making it crossfirable on a 650w power supply
Other mentions: gtx 560ti 200-220, possible to find it lower in several occasions
HD 7870 last week, the XFX and HIS 7870 was on sale for 300$. atm its still slightly above 300, but it occasionally may come back. Several models go under 300 after rebate if you do rebates.
only reason why i usually dont mention the gtx 560ti448 is that its basically like the same price as the gtx 570
oh ok thats good to hear, is there a big difference between the 7850 and 7870?
There's a pretty significant difference in performance, yes, but with that comes a difference in the price as well ($50+). So really it's just whatever you're comfortable paying. Any of those cards will perform wonders compared to your current 5770.
like i said in my quote above, it will similarly work like a 7850, but consume more power. up there is also the code which puts the card down to 200$, which is a nice price.(it comes on sale every once in awhile). if you want to take a flash chance as well, it is possible to turn a 6950 for a 6970 through a flashing, but doing so has a chance of bricking it.