and I am very excited at the prospect. I can't help you if you decide to go down the build route but let me add my two cents.
First off, I totted up the total cost of my components, and then went on several websites to try and match as close as I could to the same spec. My own build came out cheaper on every site, and by a good chunk too (that also includes the Desktop Option site - don't get anyone started on their customer service but I have found they have good prices, and I wanted the lowest quote).
Secondly, if you self build you don't get the warranty of a shop bought one, but I think I am right in saying that each component will have its own warranty anyway, just keep hold of all the receipts. Also, many shop built computer warranties become void if you tinker with the PC, something I very much dislike and was the driving force behind me wanting to build myself, plus I wanted to build specifically a very quiet PC.
Last thing to consider is the fact that if you build yourself, not only is it exactly to your spec, but you are sure of the brand and quality you are putting in the machine. I'm sure many manufacturers cut costs by including the odd no-name brand to save a few pence. Also, by building yourself, you will gain a far better understanding (if you haven't already) of how your computer hardware works, which is always a bonus.