personally i get all my parts from Novatech at
www.novatech.co.uk I have built about four machines although about three years ago. PC world is an absolute last resort and OK if you know exactlty what you need, dont rely on the staff! Well thats my experience anyway, I bought a book with easy to follow photos and used is as a guide. I found building the machines easy, the tricky part was loading the operating system and other software. I built the PC's in probably an hour but the software can take best part of a day, unless you are sat there doing nothing else. One other tip is not to canabalise the machine you currently use so that you can download all the drivers and help info that you need before your new PC is operational.