Leopard is lovely, but Microsoft is a LONG way from being doomed by anything Apple does for the foreseeable future. There is a slow trickle to the Mac platform but that's all it's going to be.
I'd stay well away from Vista, which was essentially forced on a user base who didn't want it. Typical Microsoft really. We'll only make DX10 for Vista so in three years you HAVE to buy our product if you want to play games.
If you want a Windows platform, get XP. Be quick though since I hear MS will not be selling any more licenses for it early next year. Their stealth way of taking away your choice and forcing you into Vista.
Linux is a good system, but be prepared to fiddle with it.
As for the hardware, get a PC and you can run Linix AND XP. Use the latter for your games and the former for everything else.
Or if you want a Mac, you can run XP as well as Mac OSX and get the best of both worlds there as well. However, the graphics cards in Macs tend to be a litlle behind the curve when compared with gaming PCs.
And of course, you can always use an Amiga because there is nothing like that smell from old power supplies and circuit boards. You know the one. The 'is it going to catch fire?' kind of smell.
