I'm in the process of building 10 machines with a mate for his company. Sadly for us, XP is not an option, as it ceases to be supported to most users in April this year (ie you get security updates, but none of the remaining hotfixes, which are available if you buy the support contract thingy (but that's not really an option for us, given our budget).
This puts us in the impossible position of *having* to use Vista, with which neither of us is as accustomed to as we are with XP. Some things on Vista are okay, but personally I find the copy function over network shares or between physical drives seems to be really slow, and a lot of the visual features seem completely superfluous to me. Then again, I use XP stripped down to look more like 2000, so I guess it was always going to be a shock!
I also use a Mac with work, and there are some things which OSX has which are much, much nicer than Windows OSes (and some things which are MUCH more horrible, like the diabolical search capabilities of the Mac - these are truly AWFUL in comparison to Win2k upwards).
I have to say, I was very impressed with Suse 10.2 when I tried it a couple of years ago. Most of the Linux distros I've used have been in many ways superior to both OSX and Windows versions (and in some ways more hassle). On the whole, I would go for these if given the choice, and if the games I play at home also ran in that environment!!
On a separate note, I'm surprised you bought a Dell, Wez, having built your own machines in the past! I've personally had so many bad experiences with Dell through various workplaces, and for most of the machines they build, I disagree with the specs they choose (and the price they come to, on the whole), that I wouldn't touch one with anyone else's money, let alone my own!!
Roo