Check this site out
Its a fantastic online computer builder.
You select each component at each step, eg. case, processor, motherboard, ram
Makes everything compaitable, then you pay for the components and they build it free. Its great my cousin got his delivered the other day he said its fantastic.
Currently only delevers to UK, because there based in the UK but i hear there making a US one, when they get a few more customers.
By the way its a php builder tool, thought id say summat which represents my name. lol