Wang8's idea is reasonable but the only trouble is that you will always need to turn on the computer with two network cards for the internet connection. A hub will be a little bit expensive but will be more flexible for further use......
to socery:
cheapest way of sharing highspeed internet with 2 PCs:
1) two ethernet card in one pc
2) one ehternet card in another pc
3) connect DSL moden with the PC which has two network cards
4) using a crossover patch cable connecting two PCs
got it? after the the hardware setup, you still need to do some software settings.