I have to say that when I think about it, I've had more stability issues with OS X than I ever had with XP. It's mostly due to programs just quitting on me, which is really annoying. Also for some reason it will just refuse to sleep or shut down so I have to force it down by holding the power button. It happens at least a couple times a week. Luckily it's never been with the Office programs where I've lost data, but it does make me nervous enough to save about every 5 minutes. With XP I never had programs just quit for no apparent reason and I never had BSOD issues except for when I was tinkering with things I shouldn't have been. I still like OS X better though.
I just finished installing Vista on my 4 1/2 year old Alienware desktop last night and so far it runs like a champ. Installed programs and games, all running fine. Although, at the time I bought that computer it was "top of the line" with a 3GHz Pentium 4, 1GB of RAM, Radeon 9700 graphics card, and Soundblaster Platinum EX sound card. I was planning to use it for an "entertainment PC" or something like that, but I might just sell it while it may be worth something to someone.