T.H.,
I'm jumping threads here (Sorry to OP, and thanks for the info in my other thread), but I was wondering about just this thing: re-using parts. I was thinking about re-using my HD's (I have a small 75GB that will work fine for apps & the OS), case, and OS (XP Pro). Is there any cavats you know of to doing this? Does an old-ish (5 years) HD work well with new boards? Will XP allow me to load it onto another MOBO? From some reason, I seem to have memory of that being a problem
That won't be a problem. Drives from 1986 (if I recall correctly) right up until the last few years were all PATA (also known as, and often called, IDE), and the newer ones, from the last year or two, are SATA. Don't worry about what the acronyms mean, unless you want to for whatever reason. You will, however, need to know the terms when picking your motherboard. Just make sure the motherboard you get has an IDE controller onboard (which most still do, including the P5N-D), and you'll be fine.
There are quite obnoxious problems when trying to install old operating systems, like the original, retail XP disks for example, on new SATA drives, because the drivers hadn't been included by MS in the OS yet, but not the other way around, that I'm aware of. If you run into this problem, you'll have to slipstream the drivers into the installation disk, which would be a topic for another thread. Maybe this is what you're thinking of?
As far as whether or not XP will "let you" install it to another system... to be quite honest, I really don't know. I started slipstreaming service packs into, and stripping the aggressive registration/activation shit ouf of, my retail copy of XP years ago when MS started that whole charade, and I have never really dealt with it, beyond those first few days when I didn't understand wtf was going on. The side-effect of this is that I also don't get updates for XP, except for the service packs, like I mentioned, but I have yet to experience a
single problem with that.
Sidenote: I do recommend doing a full format before you install the operating system, and running chkdsk afterwards, since 5 years is getting pretty old (lifespan-wise) if you use your drives extensively...data loss sucks infinitely more than anything else that can go wrong with a computer

Always better safe than sorry, in my book.