Well, as much as I love my Mac, the operating system doesn't have that large a selection of games available. (Altho that's changing, EA is moving to develop for Macs now, and other MMOs are going to the mac such as Eve, so, there's hope)
However, since you're not asking about Macs I'll recommend a Dell. They seem fairly reliable, it's just they have this nasty thing on them called Windows. Beware Vista.