Okay, I've gotta build a new computer, it's been too long since my last one crashed (water got on mobo, don't ask).

Currently, all I have to work with is my Power Supply, it's a 350W power supply, hard drives, 500gig and a 120gig, and I guess I can continue using a few of my fans and my computer case. This is my first build from scratch although I've switched out mobo's and every component before.

What I'm looking for is something to run the ubuntu linux distro on. What I need new is a mother board, ram, CPU, video card, etc.

I don't really know how much I want to spend, although I'd prefer it to be not that expensive. I would like to use the AM2 socket unless someone suggests any different... I don't really know much of a difference between processors so if anyone has suggestions let me know. Also, I only feel like I'm going to need no more than 2 gigs of ram.

Really, I don't know much about any computer components... I figure I could just let you guys know what I'm going to be using this PC for and maybe you guys could help me choose parts.

First, what I use it for:
Downloading and listening to music. I had my old PC hooked up to surround sound in my room, I'd appreciate it if this were possible too.
Going on the internet, of course any computer can do this really.
Sometimes I use CAD, not often anymore but I can see me using it again in the future.
I like having dual monitors, makes it so I can put windows media player on my secondary monitor and still use the primary monitor for browsing the web.
Lastly I use it for schoolwork sometimes.

Mainly, it's music though.

I like my computers very fast, not necessarily from tons of hardware, but I use ONLY the absolutely necessary programs to keep it fast. Anything else I should look into for this?

Sorry if this post seems kinda sloppy, my girlfriend is annoying me at the moment craving attention.