vsp,
there is always another choice. You could buy an old, used system. That doesn't always seem like the greatest choice when upgrading, but, when
downgrading, it might be ideal. You can get ahold of p200's for about $100US (at least, here in Minneapolis), and then all you need is a KVM switch (maybe another $50US). I work at a used computer store, and some friends took a couple of machines that were going to be recycled and they use them for just that purpose (as well as file storage for mp3's, movies, etc...). Check around, it might be easier than underclocking your main system.
Here is one of the 985 items found on ebay.
good luck.