Don't forget the proper tools!!
To help yourself out, and to avoid the hasssle, and money spent on fried components, invest 30 bucks(Radio Shack, yes this is a plug), and get a Computer Maintenance Tool Kit. Has Anti-static mat and wrist strap, all the screwdrivers you could need, heat sink grease, and Chip Tweezers. It's worth the money.
Also, b-4 you buy all the parts brand new, check at second hand stores. A NIC cost 20 bucks brand new, but a used one cost 5. A Brand new 40 GB HD, 90 Bucks, used 10. See how much you can save....
... and yet more thoughts
In the UK certainly, we have Computer Fairs (Just do a Google search). Go to one of those, preferably knowing what you want and how much it costs from places like dabs.com (did I just do that wrong?) or Jungle. Computer fairs are usually a few £ cheaper. The end of the day can be a good time to go, especially if business was slow: haggle. But you may be saying to yourself, 'surely I'll never see this guy again'. Not true. They make their living at these places and most have bona fide parts supply companies. Get a receipt. If the product doesn't work, the guy who sold it to you WILL replace it, because despite the apparent casual nature of computer fairs, the vendors do build up a reputation and are keen to maintain it. Much of the stuff they sell does not come in a box!
Best of luck people.