That depends on your budget. In order of expense/performance
R7 260x - $100
R7 265 - $120
GTX 750ti - $140
R9 270 - $150
R9 270x - $170
R9 280 - $180
GTX 960 - $200
R9 280x - $220 (This is the one I would choose)
R9 290 - $280
R9 290x - $320 (I would chose the GTX 970 over this)
GTX 970 - $330 (Best card for the money right now, regardless of only having 3.5GB of usable memory)
GTX 980 - $530 (The best consumer gaming card there is right now)
GTX Titan Black -$? (Meant for rendering and workloads, but yolo)
Really you should upgrade your whole system. It's about that time. If you would like to start with your new graphics card that's cool. Here is a list of prospective new platforms you could jump to listed in order of performance/budget:
Gigabyte GA-F2A88XM-D3H Motherboard + Athlon X4 860K (would require a graphics card, get an APU if you cannot afford a graphics card)
Asus H81M-Plus + Pentium 3440
Asus H81M-Plus or ASRock H97M PRO4 + Core i3
Asus M5A97 R2.0 Motherboard + FX 6300 + Cooler Master 212 Evo Cooler (should be overclocked to be considered worth buying)
ASRock H97M PRO4 Motherboard + Core i5
ASRock Z97M Pro4 MOtherboard + Core i5 4690k + Cooler Master 212 Evo Cooler (The crowned king of gaming meant to be overclocked, a very very capable business platform)
And then there are enthusiast level choices involving the i7.
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