People have had problems with both the Airport Express and the Airport Extreme base station. I have an Extreme and my MacBook won't play nice with it unless it's in the same room, although my iBook and iMac will work with it anywhere.
But Apple is introducing
a new Extreme that uses the new "n" wireless technology. The Airport cards in the new MacBooks are apparently designed to work with this new technology — you'll be able to enable them to "n" later. I'm not a techie, so I don't really know much about that. If you scroll through the Internet and Networking forums for the MacBook you'll find people discussing the issue.
The MacBook will come with Quicktime, which is OK for watching clips. I use
VLC myself, which you can download for free. It plays pretty much everything, including stuff Quicktime won't, and has more features.
iMac G5 Mac OS X (10.4.2)