-Your price range kinda points towards having a browse at local used cards as well.
I have 2 systems with single hd-4870 cards. They play pretty much every game out there at 1080p with decent framerates. They are a common card to see for sale used when people upgrade. $50-$70 for a used 4870 can't be beat.
The 4870 is a bit faster than the 5770 and 6770, it is closer to the 280/285, 460se, 5830 and 6790 in speed. The drawback is the 4870 is as large as the 6970 or 570 and uses almost the same amount of power.
There is a risk with used, and you should see it working first and playing some new games.
-Some other good used cards,, the geforce 275 to 285, 460, 5830-5870.
some cards that usually have cooling issues,,,8800gtx, 295, 4870x2, geforce480, 590. The 9800gx2, and 4850x2 also have 2 gpus.
Be careful with those cards used. Make sure if you do check one out that both gpus are working in the driver menu, both the 4870x2 and 295 are known to still work ok on one gpu, but it makes the card the same as the single gpu card in that mode.
-some used sli or crossfire pairs can be good deals if you have a crossfire or sli board. something like a pair of 260's or 4870's or 4850's are pretty good too.