See there is loads of advice on this one lol
Couple of questions before I would advise you
1. How far are the PC's away from each other (same room or different ?
2. Does the main PC spend most of it's time hooked up to the net and second one for occassional use ?
3. Are the PC's using the same Operating system ? If so what are they running ?
Most people think that they buy the cables etc. plug in and everything is hunky dorey. Sometimes it will go right sometimes take for ever to set them up, so be prepared to do it when you have time and not rush it.
Wireless is the up and coming thing to do but perfer the method someone else mentioned.
2 network cards, a crossover network cable(not sure why you would need 2) Before buying the network cards check to see if you already have a network port on the PC as most motherboard offer 1 .... if so then it's just the cable you need which would cost you anything from a tenner upwards depending on the length.
Once you have that all sorted you will need to set up the network, do this off line and make sure the firewall is either configured or switched off or you won't get a thing. Once you have got each computer to recognise the other then configure your firewall to allow the second PC and then connect to the net.
Not sure why you would need to bother with a router unless you want to use the second PC without turning the 1st one on.
For very little money £14.99 you get everything you need in a nice kit
http://www.maplin.co.uk/Module.aspx?...534&doy=11m10D
Wireless may be a bad idea if you don't configure it properly, people will beable to use your connection to surf the net .... why pay for someone else to slow your connection speed down.