In order:

1: If you purchase an additional license (for each additional system) with it's own unique product key, you can install XP on more than one system with the same CD. However, this doesn't apply to OEM and volume license editions like you get with a computer.

2: Firewire and USB were not designed for that sort of use, so while it's possible, you'll tend to have much better luck with a simple crossover ethernet cable. It's almost impossible to find a system sold these days without a 10/100 ethernet controller built in. If you are looking at connecting systems over distances more than a few feet, consider getting a router and just some regular (non-crossover) ethernet cable. Then the tricky part comes in, in setting up Windows file sharing. There are countless guides on how to do this that are only a quick google search away, so I won't duplicate that effort here.