Questions regarding new network setup

I just recently got a new linksys wireless router (WRT54GS) and it's setup and running. Have the MAC filter enabled and using WPA security. Currently my parents are paying for 4 IP addresses from our ISP (used to be on a hub). With this router, will i need to continue to pay for the 4 or can I dump 3? If so, will the network performance be any different then from the past? Also, the signal and the quality is varying between 73% to 80%. The wireless computer is in a room right next to the room where the router is, so is this good or bad? If bad, will I need to get an extender (2 computers are on the second floor of the house while the router is downstairs)?

Your help is appreciated :)

dump them, they dont give you any extra speed, they just charge you for the use of an IP
going wireless will kill computer to computer speeds, everything else will be about the same.