I'm not going to pretend to know anything about programming, so I'm sure a Mac may very well be useless for Dave.
I agree that XP Pro was head and shoulders above my previous OS, Windows 98 (which my girlfriend still runs on her computer...shudder...). I just much prefer Mac OS 10.3 Panther. I'm in luv!
Tuzenbach, if you're upgrading to play games at all, get Windows XP and call it a day. Apple's games catalog is notoriously sparse (but we have World of Warcraft!). Otherwise, think about saving some extra cash and getting a Mac Mini for reasons mentioned above. Feel free to email me any questions as well.