Barry's right : not much point in trying...
First you would need to find/buy USB NIC's, which in most cases are more expensive than PCI cards, and then you would probably come to the conlusion that all of the USB NIC's on the market today are USB 1.1 and thus limited at roughly 1Mb/s instead of 10 or 100Mb/s...
Same story for the firewire solutions (except the speed limitations)...
In short : get a network starter kit with 2 Nics & a crossover cable and you will have a cheaper, faster solution.