I've had luck gaming with XP Home on my main rig. Networking is more complicated and configurable with XP Pro but you may not need all that. You get to hunt down and turn off more unnecessary servers with XP Pro. I think I still have a motd server on my XP Pro rig and it wasn't really obvious how to turn it off.
XP Home is cheaper and works just fine with AH, AH2, and AH H2H. YMMV, but I haven't yet found a reason to wish I'd put XP Pro on my game rig.