you'll need one router, and an NIC (network interface card) in each computer
if you really want to go wireless, theres a lot of things to consider first.
do you have a microwave oven in your house? if you do, it is liable to cause interference if you get a wireless network which works by the 802.11b standard (2.4GHz). the same applies to any bluetooth devices you have in your house, or near by for that matter. as for the network cards, I highly doubt that you'll have wireless NIC's installed already, so you'll need to buy and install those. just make sure that all the equipment that you buy works on the same frequency (802.11g should be fine, but can someone confirm that please, i'm not entirely sure). as for the AP (access point), the router/hub will most probably have the AP on it (click here i think that should work fine for the AP/router)
how old are the computers? if they are relatively new, theres a good chance they will have an ethernet port already in there, which will make setting up a wired network a lot easier. as for setting up a wired network, you'll need a router/modem/hub (most commonly you'll find it is a 4 port hub). bare in mind that you will need a crossover cable running between the router and the PC's (if your not sure what this means, i'll post again explaining)
hope this helps
if i've missed anything, someone correct me please, i'm still pretty new to this, and this seems likes a good place to check if my knowledge is correct before i make a really big mistake :S