A router would be easier. A crossover cable just simply connects the two pc's to one another. Basically what you do is connect one pc to the ntl box and connect the other pc to the first one via the crossover cable. The downside to this is that the main computer 1) has to have 2 network cards (not a big deal really) and 2) has to be on for the other pc to access the internet.
Whereas, if you have a router, wired or wireless, it's your choice really, you connect the router to the ntl box and then each pc has an independent connection to the router, and an independent connection to the internet. It's a lot easier and not hard at all to setup. The downside though is that routers can run from $50-$100, plus if you go wireless you have to buy wireless network adapters for each computer, which can run as much as $80 apeice as well.
Hope I helped clear some things up for you.