The best GPU you can possibly get for that PSU would be a Radeon 7750.
It'll do well, you can get very playable framerates on low-medium settings, depending if the CPU is fine as well of course.
Anything other than a 7750 will need a stronger PSU, but a 7750 should do fine depending on what games you'd like to play. I doubt you'll get away with some of the newest games.
7750 -£80 http://www.amazon.co.uk/gp/product/B00761YFPW/ref=pd_lpo_k2_dp_sr_1?pf_rd_p=103612307&pf_rd_s=lpo-top-stripe&pf_rd_t=201&pf_rd_i=B007EME3XW&pf_rd_m=A3P5ROKL5A1OLE&pf_rd_r=0CA7BP3PZJYAZPDMYSJA