What makes a computer?

[phoenix_prose]

I'm wanting to get a new computer, but I thought that instead of buying a pre-fab one, I'd buy customised parts and get a friend of mine to stick'em together for me. So...what do I need? Any recommendations? I want a decent sound card, good graphics...basically I'll be using it for internet, music and gaming, so I want something that'll support the likes of Dungeon Siege, but at the same time I don't really need the best of the best, know what I mean?

All help will be rewarded with wooddust and e-cookies!