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First, do it! It's a lot of fun.
I was going to share my experiences, (I've built 6) but Zaphrod did such a stellar job of describing the process that I'll skip the build part. Good job, Zaphrod!
I would add a few things:
1. The process, the first time, will take about three or four hours. Get comfortable.
2. Really do read the motherboard manual. Keep it close.
3. I do have some hardware recommendations. Bear in mind I'm a real cheapskate.
ECS K7S5A or K7S5A pro.
Athlon 2000+ or for the real cheap...Duron 1.3GHz
The ECS board has onboard NIC and sound, so you save there.
NVidia video cards...I bought a generic 64MB($40)with the TV-out option, which can be useful sometimes.
Get a decent modem, if you're on dial-up. Winmodems really put the hurt on your system's processing power. Don't know why.
Two places I spent the money...Got the Enermax "Whisper" series power supply and the Zalman "Silent" series CPU cooler. I can't stand noise, and these two products did such a good job I don't have to use case fans. The machine is VERY quiet.
You'll notice with hard drives there is a cutoff point, pricewise, where getting more space suddenly costs MUCH more. You should be able to find an 80 GB for around $100 these days.
You should be able to build the entire box for no more than $400.
I do recommend getting a GOOD monitor, as that will be with you for quite a while. If you must have LCD, don't but online. Actually look at what you're getting as the quality varies a lot.
Get a good keyboard and mouse. These too will be with you.
Many people here have posted some really good suggestions. Cable ties, easy open cases, etc.
And one last word...ignore the Mac trolls. They're just jealous. Building your own Mac is fun but economically not worth it...and it's against the Apple/OSX EULA.