Apple, then get bootcamp, then a *shudder* windows xp disk. Perhaps Vista if your inclined. At any rate the new apples are powerhouses, especially the macbook pro's. Hell my Macbook Pro is 2 1/2 yrs old and was still able to run new games with little problem.
Of course atm, i've switched to my gamer cpu which has a little bit better specs than the macbook.
For a good gaming rig go with
http://www.falcon-nw.com/
but if you get one of those, get two. So that way I can have one as well
I wouldn't necessarily recommend dell or the others cause it's usually a pain to upgrade them. BUT I hear they make some great gaming rigs. I think HP has a great one at atm, but I can't recall the name.
http://alienware.com/ is another
As far as your questions, for the OS, my friend says Vista and it's direct x 10 makes a superb difference. While he hates it (well all things microsoft) he normally hosts using his powermac (with bootcamp) to run Vista. It generally runs pretty good, but Vista does some crazy s**t with firewalls and blocking, and usually it takes us an hour or so to get a game running due to vista.
I use XP because I don't think my components are compatible on my gamer. Xp for me is fairly stable, but then all I do is Game on the computer.
Mac OSX.....oh yes I did! I've played Spores on here and another game or two. OSX I think is probably the most stable, but game makers still aren't putting out that many games that will work with OSX, they are getting better though.
As far as the components I haven't been keeping up with them myself so I can't really recommend anything except saying you want 2 gigs or more of RAM and at 100 gigs of HD space total. I'm not sure how many games your planning on putting on, but I have an 80 gig on my gamer, and I think I get about 4-5 games on there, and windows from what I've seen cries and goes to s**t if it has less than a gig or two free space.
For the motherboard I like ASUS, but some don't. IMO spend your money on the motherboard, get one thats easily upgradeable, and has a lot of room for expansion (can handle more than what your putting in).
) so what would you recommend in 
The PCs we bought at work recently cost just over £500 a piece... for development purposes.
