I'd consider at least the 7850 GPU and a decent PSU (if you don't already have a good one). Antec/Corsair/Seasonic/PCP&C are all good brands to consider when buying a quality PSU. You would need at least 450w for the 7850 GPU and probably should look for at least 500w.
got a 550 psu,nothing great just a generic £25 cheap job to replace a broken part.
i was looking at a radeon hd 7850 but how does that work with PCIe 3.0 (x16) is it bckward compatble?