First of all, I feel games work out better with a PC, so try to avoid that as much as possible.
Second, I think the most obvious reason would be that the Macintosh OS, especially OS X 10.1 and even 10.2, has a much more attractive appearance, than Windows, as well as more well designed software. Tell her how easy it is to use Mail. I have rarely (and I mean rarely) had Mail crash on me. If she will need something to communicate, much like a Chat room, make sure she has a .Mac username, or an AOL username. Then tell her about iChat and how simple iChat works. If she has a busy schedule, tell her about iCal and your personal opinions about iCal.
Third, I would put some background about your personal experience, for whatever applications you feel she will use most. And, of course, act positive when describing how great a Mac can be.