What's the make & model of your PSU? Assuming it's a half decent brand, here's a few suggestions:
Pure bang-for-buck, there is a super-cheap R9 290 going at the moment on Newegg:
R9 290, $210 INC rebate, $230 without rebate: http://www.newegg.com/Product/Product.aspx?Item=N82E16814202043&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-PCPartPicker,%20LLC-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=
That is approaching the speed of a $300+ GTX 970 for a super-cheap price, BUT, it's a stock-cooled 290, it will run hot and it will run load. In terms of FPS for your dollar, that's an absolute bargain, but will be hot & load, and needs your PSU to be a decent one.
Otherwise you can look at a 380X for $230:
http://pcpartpicker.com/mr/newegg/gigabyte-video-card-gvr938xg1gaming4g
A $310 budget gets you into R9 390 or GTX 970 territory, which is a decent step up from the 380X, so depends how strict your budget is.
If you really want to go Nvidia - you're stuck between a GTX 960 which is under budget, or a GTX 970 which is well over budget. And there's a big performance gap between them too.