I had the same scenerio ( a voice over booth). I set up my system like this.
1) PIII 650 sitting right outside the booth. I ran a USB cable and VGA cable through a larger hole in the side. I then took a piece of 4"x"4" piece of MDF, cut it in half, clamped the halves back together and drilled a 1/2' on the center.
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2) Ran the cables through the big hole, then mounted my makeshift cable clamp on the outside, the hole filled by the cables. Next went silcone sealant into the hole.
3) The USB cable connected to a USB hub, which connected to the mouse and keyboard.
4) The VGA cable ran to a Sony 15" LCD.
That worked quite well.
I was also running a home workgroup on Windows XP Pro, so I set the PIII to remotely run my main recording machine, a P4 3ghtz. The Remote Desktop feature gives you the freedom to locate your booth and editing area away from each other, limited only by the length of a network cable instead of video and mouse cables.
For the mic and headphones, I built a couple of wallplates out of MDF with an XLR and 1/4 stereo jack on each side. Markettek or Redco Audio (
http://www.redco.com/) have those items.
Good luck with the build.