i say the first one since it has a amd 64 cpu, however the 6600gt that it has will limit u to newer games at high settings i.e fear, quake 4, so if u get that one then i suggest u update your gfx card, however the second is pretty good as well, a 3.0 ghz p4 630 cpu and a 6800xt hmm pretty good but 512 ram only? hmm
ok in my own personal opinon, u say your buget is £800 then
go the first one, sell the 6600gt on ebay, u should make at least £120 for it, then u will have at least £320 left over to buy a kickass gfx card like a 7800gtx or 1800xt pe whenh they come out, then u will be set for the next two yrs or more