Building my first PC!

[Fizzgig]

Well, I have one already, but it was bought all together. I've been looking to build a new one since mine is really starting to show its' age (had it since 2000). Now, I'm looking to spend around $500-600 dollars (is that realistic?), and a friend of mine said I should go with AMD.
I have a general idea of parts, but I'm not sure about what kind, what's compatible, etc. I'd like to be able to play some newer games (Sims 2, AOE III). I'm not planning on recycling much, probably just my monitor and speakers.
Another thing I'm interested in is video editing. I've played around with Windows Movie Maker 2 on a friends' PC, mostly because it was simple to learn and use, but I'd like to try some others, like After Effects. Of course, I'd have to learn how to use it, too, but that's another matter.
I'm not looking for top of the line, obviously, but something that'll run the latest programs without chugging. I need the advice, since I don't know much at all about putting one together.

[kram 2.0]

You may want to open up the budget a bit more before attempting to build a system that can handle video editing with any sort of ease, especially on the AMD side. My suggestion is to save up until you have at least 800 USD on budget in order to build a system that is marginally better than a manufacturer-built system.

kram

[Fizzgig]

Well, I can do $800, I was just hoping for cheaper. :|

[Kareeser]

Cheaper will mean sacrificing essentials that make a good gaming system. The things you need as a gamer are:

A good video card
Quality RAM, and enough of it
A fast CPU

On top of that, you need a motherboard, which is about $100-200

I suppose that it would be possible to build a computer with an $800 budget, and be able to play Age of Empires III, as it is not too graphics intensive, however, more spectacular effects won't be available to you in-game...