A MacBook is definitely a nice, solid way to start out. All of the MacBooks Apple sells have plenty of GHz or memory to browse the internet and listen to music. I'm not sure what the system requirements are for World of Warcraft, but it's likely the MacBook would meet them.
However, if you do not require your Mac to be a notebook, I suggest you get the
Mac Mini. While the Mini is not an all-in-one like the iMac - you need to have a monitor - it's great way to start using Macs and it's a lot cheaper - starting at $599 for 1.83 GHz and an 80GB HD.
Once you start using your Mac for more power-demanding work (if you're that kind of person you'll start building your way up to more powerful Macs. Two years ago, before I started doing a lot of graphic design, I had
this Mac. Then I needed to use it for more work, so I bought a Mac Mini. Then, I bought a MacBook Pro (1 GB RAM). When I smashed that, I bought my current black MacBook (2 GB RAM). I'm now looking into getting a
MacPro in the next year or so.