Hi Jim,
You can get an AMD quad core cheaper (£219 inc. VAT and delivery) here
http://www.cube247.co.uk/products.asp?partno=Sigma+GT5 plus it's got 8Gb of RAM. You will have to add an operating system to that price though, but it will still be cheaper than the ebay one.
Or how about something like this
http://www.cube247.co.uk/products.asp?partno=Dorado+GT4 you might want to 'upgrade' the processor to the Intel i5, again you'll have to add an operating system.
Like Matt, I used to build my own PCs, but after seeing the prices that Cube build systems for, I let them do it now. BTW I don't work for them.
Regards
Nick