Up to about 250, try a 7850. If you can get a deal on either the 7850 or 7870, those are your best bets.
Otherwise, you can try Nvidia's last generation 570/580. They are sometimes on sale for a heavy discount. Be aware that they require a lot more power to run, so while your PSU is okay for that, your cooling may need some refining (hidden costs with fans, etc).
I was thinking about the 570, I am more of an nVidia Fanboy... since so far AMD drivers are broken for me...
The 570 is 21 bucks more than 250, but I can squeeze that in.
So either the 7850 or 570.