I can't afford to build this thing at all, but I'm doing it anyway - price is the most important factor. I currently have an old dell tower (from like, 2005?) hooked up to a crappy projector (hoping to upgrade to HD one day). I'm planning to just use the old dell case, which must be ATX(?), so there's that. I imagine I'll need a bigger power supply, though it would be sweet if I didn't (the one in there is 300 watts). Also, it would be nice if I could get the thing running a few older games at low resolution (think TF2) and maybe the PS2 emulator. I'll be running Windows 7.
Onboard Optical SPDIF out would be great, otherwise I have a Turtle Beach Riviera PCI card that I'll be using.