I'd suggest upgrading the PSU now and saving up some more money for a GPU. A really nice PSU in the range you're looking at: http://www.scan.co.uk/products/450w-psu-xfx-pro-core-edition-p1-450s-u2b9-85-eff-80-plus-bronze-sli-crossfire-eps-12v-fan-atx-v231
I think that GPU prices are going to start falling some as the new nvidia cards hit the street; the GTX680 just tool over the #1 spot for single-card solutions and the rest of the product line will likely spark a price war as AMD/nVidia jockey for position in certain price ranges.
Edit: This PSU only has (1) PCIe power connector and this will limit you to something below an HD6850/GTX560 (these cards and anything higher requires two PCIe connectors). If you want to look at consider a card that requires (2) PCIe connectors, then I would recommend another PSU that has the connectors without having to use an adapter.