I have XP home on two computers, and XP Pro on my laptop. There's nothing that really strikes me as so important that Pro is needed. I can do all the tweaks I want on Home.
How much do you really want to fiddle ? I'd save some $ and choose XP Home.
you may wish to consider windows 2003 server - this is not windows 2000 with a bunch of service packs bolted on. I have not tried it myself but I have heard it recommended several times by producers I have met. I think it doesn't have the stupid XP authorization system (I could be wrong about this though).
If you don't already know, you should : XP will need you to reauthorize it after you change a few pieces of hardware (certainly the last thing you need if you're tinkering around trying to get stuff to work).
Most sane XP people will use the corporate version even if they have the legit copy. It doesn't need you to reauthorize it either but to buy this you need to be a sizeable company....