so you have 1 pc directly connected to a modem this in turn share the net wirelessly with other computers? somewhere down the line you want to link a laptop to a computer that is using an internet connection shared by main pc?
I really do not know if this can be done i dont know if a shared connection can be shared again.You will need crossover cable and not straight to network these 2 computers anyway.The wireless access point is of no use without a wireless connection in the last machine you will still be trying to share an already shared connection
I would advise a router at the begining may as well use a wireless one and then maybe the connection can be shared or you could just buy a belkin cardbus wireless adapter for 20 quid from argos

(but of course you dont want to do this yet)