if its 6 months old, chances are its not PCI 2.0 but since none of the cards out there really use the extra bandwidth (ok, so the new midranges support it, but its not exactly like they saturate the bandwidth of pcie x16) http://www.overclockers.co.uk/showproduct.php?prodid=GX...
might be interesting to you, its a 3850 with 512 mb vram or for slightly above your budget you could find a 3870 of some sort (don't hope for an 8800 gt, those things' price is way above your budget)
alternately, you could go below your budget and get a 3850 with 256, but that will start to choke at higher resolutions nowadays
I think get the 8800 gt, since it outperforms the 3870 in most games, is from a reputable company, and is only a few £ more (i mean, if you are going to go over budge by that much, why not go a little further...? )
you should get the 8800gt if they are that close in price. the new ATI cards have AA issues, and the FPS gets killed when you enable even 4x AA. both companies are reputable, and both cards are great, i would just lean towards the 8800gt 1. because i just got mine and its sick 2. less issues with drivers and such
the only time i would recommend the 3870 is if you wanted to eventually do a crossfire setup, in which it really does pretty well compared to SLI