According to this
GPU hierarchy chart Nothing above a HD5770/6770/(reservation towards a GTX750Ti due to being current gen however is very power efficient) though you'll need a reliable 450W unit to power your entire rig and no matter what you do you're going to see a bottleneck on your CPU side. If you could pass on the full systems specs we'd be able to understand if your system is capable of accepting a GPU upgrade in terms of PCI Express connectivity/lanes.
Simple speaking, a GPU cannot be recommended at 250W PSU wattage and the CPU will bottleneck your system though overclocking may remove that factor
a little.
Please include information in the body of the thread instead of the title bar. There just isn't enough room to include everything in the title area.
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Lutfij