Try PC Advisor magazine or
http://www.pcadvisor.co.uk they do very good reviews and have top 10 from budget to super pc. Personally wouldn't touch Dell but then again it is personal preference as they say YOU pays your money etc......
I've read somewhere that ideal would be 700mhz + 32mb video card + 256mb mem. There have been issues with the AMD and NVIDEA Geforce card (guillemot prophet uses) but i think there are patches for it
If not done already try contacting people like SMC direct
http://www.smcdirect.co.uk or Novatech
http://www.novatech.com (I think)http://dabs.co.uk they build to your spec. Have fun