Yup, your gonna need a wireless router to replace the wired one you have. YOu can still connect up the office PC's to the wireless ones using wires aswell ,so you won't need to get a hole bunch of wireless LAN cards for those.

At my work everything is done by cable, but they just plugged a wireless router in downstairs whci controls one PC by wireless, 1 laptop by wireless and we have 2 other PC's plugged in with cat 5 cable aswell...... Think it is a Netgear one, tho I don't know which model...