Motherboards range in price depending on how nice it is (obviously). I have a K8N motherboard by Asus, and it was about 85 buckaroos. Then there's the video card, and I doubt you'll want it... I'm really trying to persuade you not to get one; you won't like it in the long run if you're looking for some nice looking games. Here's a scenario: I love the game Tribes: Vengeance, and whenever I played it with my current setup with the exception of the graphics card, I had to turn it down to 640x480 resolution and low details all around except for the fogging distance (how far you can see) which I had set at high so I could snipe. This gave me aroud 20 FPS... Now, I replaced that card with a 6600 GT and I can now play T:V with everything on high and fogging distance on ultra-high getting about 35-60 FPS! That's a huge difference for about a hundred bucks. A case will cost you about 40 bucks for a mediocre one, but I got a cheap one which was 35 bucks and I like it pretty well, though it's a bit flimsy when all the panels are off. If you're going to build a new computer (which is basically installing a new motherboard in a new case and then switching everything over), then get one of those anti-static mats before you do anything. Other than that, just follow the instructions with everything and you should be fine if you have an extra pair of hands around, preferably good with electronic equipment. It's not too bad, but when you're pressing the heatsink on the CPU it's a bit scary because you have to put pressure on it... It got my heart beating rather fast...
Oh, but why am I telling you this now? If you decide to go through with this, then I'm sure some of the more knowledgeable fellows around here will be more than happy to help you out. I'm just a newbie to this whole computer-building business...
Give yourself some time to think about it though... They are expensive to build if you get nice parts, and they're fragile parts, so if you damage one, you're pretty much screwed.
-Jamxx