build your own man thats what i did, it aint that hard. i got my system on dual boot vista 64/ xp. if you want to buy one look at the usual places ccl, ebuyer etc they usualy sell it with out the os which is about 100 quid cheaper
You dont say what you will be using it for but if its for games then dont bother as most games at present are not written for multi-core processors, those that are are for 2 core only.