Obtaining 54 speed in B-Wireless network

Hi, I have a Linksys B Wireless network. My kid needs access in his room, but he wants more than the 11mpbs a B card can provide. So He got a G card, compatible with the B Network, but he is getting only a max. of 11mbps.
Is there a way to obtain a higher speed? or is this a matter of upgrading the router to a G type?
Thx