Everything you ever needed to know about PC's:
1: Don't buy them pre-made from large computer retailers because they'll offer you a cheap PC with a single up to date component, tell you it's up to date whilst stinging back on everything else making the entire PC useless.
That's it.
Just buy the individual components yourself. GPU (Nvidia, ATI), CPU (pentium, AMD), RAM (if you go budget & get 512mb, get a single 512mb chip and not 2x 256mb chips so you can easily upgrade to 1GB later on). Suitable case & fan (depending on CPU choice), and a decent compatible motherboard. Salavage old parts such as DVD-ROM, Disk Drive, Moniter, Keyboard, Mouse, Games, Opperating System, Hard Disk, Pink Fluffy Dice + whatever from your old machine. Then either build it yourself or take it into a shop and pay them to build it for you.
By the way, building a PC is like Lego. The only exception being pissing around with your motherboard which is a proper pain in the ass depending on which motherboard you buy. If you intend on building it yourself then make damn sure that the BIOS on your motherboard is user friendly. Preferably with a component search feature.
Don't know what components to buy? If you are reading this message then you have access to the Internet. Not just an infinite supply of porn, but also full of advice and reviews on an infinite array of subjects from cooking to PC hardware too! Go nuts.