Hmm, despite this sounding like a cool project, I think I've decided against it. All of the upgrades I'd need to make it worthwhile (low-pro video card, PSU, thermal paste, heatsink?) would cost around $100 extra. Assuming that the computer is is worth $150 of the price, that puts the system at $250. For that price I could get a cheap barebones kit from Tiger that would perform better than the Pentium D.
Fun to think about though, and I appreciate all your advice.