It does get a little more complicated when you have to pick the MoBo, but not much. Just make sure it supports the features that you must have. USB 2.0 is nice, and you can get by with a board that has integrated audio/video, but you'll be happier with separate cards for those things. In the interest of a budget, you may want to get integrated audio/video and upgrade later, as those cards will add to the overall price of the system.
As long as you get a case that has at least 2 internal 3 1/2" bays, 3 external 5 1/4" bays and two internal 3 1/2" bays you are really looking at a pretty face for your computer. It has to fit the board you want also, and most of the cases are for ATX size boards. You will probabaly want at least a 350 watt power supply as well.
The cases from Antec are nicely finished and have the requirements for most people. The Sonata is nice with a 380 watt power supply and is very roomy to work in.
Here's a link to the Sonata case.
http://www.tigerdirect.com/applications ... =A453-2012
As far as processors go, I don't think that you need the Althon 64/fx, but if you can swing it, you'd have the best processor on the planet. It has features that will optimize video and other digital media processing. Unless you are a budding Spielberg also, it would be severe overkill. The Opteron is for servers.
I know that some people are AMD guys and others are Intel guys, but either a P4 with HyperThreading or an AMD Athlon XP will be a great processor. The Athlon is a little cheaper for similar performance, but the P4 is top of the line also. You really can't go wrong.
Either way you go, just be sure you overbuy - meaning buy a little more processor than you need now - but do not buy the latest and greatest as the price premium is too great for someone like yourself on a budget. I would say a P4 at 2.8 Ghz (about $200), or a Athlon XP 2800+ (about $135.00) would be perfect. That Althon 64fx is running about $800, so you can see what I mean.
Go with high quality (Kingston, Crucial) RAM at a higher speed, like DDR 2700 or DDR 3200 if you can swing it. You'll be glad you did.
Sorry this is so long, but there is a lot of thought that has to go into building the computer. In fact, it is more planning than assembly, but if you do it right, you will really love it.
You can email me at mailto:firstname.lastname@example.org
if you want more info without filling this thread.