Steve it's fairly easy and I have done it a number of times. However the risk is that should one component become damaged or not work duringthe build you'll need some fairly high level skills to fix it. Remember that you won't also have the same warranty backup. I've just bought a high end PC fully made up with a two year warranty that when I priced the parts up I would of had a saving of £50. It wasn't worth the hassle and I've got better piece of mind that all the parts are compatable and covered under a good warranty. Think carefully.