You can use any router. Don't be tempted to buy more of Apple's overpriced lumps of acrylic than you have to. I have a Linksys WRT54G and it works pretty well. Also works well with my dad's Belkin.
EDIT: You plug the router in the back of your STB. You turn everything off. You power on your STB, and then your router, and then your computer. Onec it is up, you'll have to add your router to your account (called "provisioning" I think), by loading up a browser and going to
http://start.ntl You'll need you PID for it. Then shut everything down, load up your STB, then router, then computer, in that order, and enjoy. Everything should be configured automatically.