I'd be looking at a GTX 760, AMD Cards have had huge Price sours and thus puts Nvidia in the best place to be. The GTX 760 is a very good graphics card, It's basically a GTX 670 and almost runs at 200GB/s but can run over if overclocked, a very good card to have for the price. 2GB 256-Bit. If you can get a 7950 for under $300 then get one, great performer and can be overclocked to 7970 level. It is 3GB 384-Bit and a very decent performer. Just depends on you Power supply to see if you can use any of those cards. The GTX 760 Is 175 watts and the 7950 is 200w
770 or R9 280 > 760 or 7750
It depends on how long you're willing to wait, if you're impatient like me and can't wait possibly a couple more weeks for R9 GPUs to drop in price (I'm somehow waiting anyways), the 760 is the one I would recommend that's in your price range.
The R9 280 was listed at 300$ MSRP before the litemining supply/demand kicked in and raised prices, and depending on monitor resolution/games you're playing is better than the 770. If you can grab a used 770, that should get you into the 250-300$ range. We have no idea when the prices will normalize though, so it really depends on how soon you're building your new rig.
Monitor resolution plays a pretty heavy role in GPU selection though, are you running on 1080p? If you're running any higher of a resolution a 280 or better GPU is really recommended, if you're at 1080p or lower it generally doesn't make too much of a difference.
Also keep in mind that the better GPU you get now, the longer it will take before you will need to upgrade again. If you aren't too picky with how good graphics are in your games, you can get by an upwards of 3+ years on a decent GPU. If you get the bare minimum that your preferences for graphical quality can take, you're likely to upgrade much sooner and have to buy a completely different card. What kind of FPS/Games/Settings do you want to run some games on?