Well you'll only be able to run Logic Pro on a Mac (Logic 5 was the last for Windows native executables), but if you get a decent Mac you can use bootcamp to boot it into Windows to play games. Otherwise persuade him to use Reason 4
Many games are now coming out on the Mac natively, however it depends what sort of game he wants to play. For example FSX won't run nice, but X-Plane will, you can get Football Manager for both Windows and Mac.
I use Windows at home (for FSX) and work, but most of my home stuff is on Linux and Mac - I find programming easier on a 'nix platform for some reason. My gaming machine (the Windows one) is a reasonably serious rig and I'd recommend Windows over Mac for that area. For everything else (music, video, programming, document writing, website building, anything else...) I'd recommend the easier, 'cleaner' Mac OSx or Fedora, but then I've not yet put my Windows7 CD in to have a go...