Slow Internet when using wireless card

I am using a Realtek RTL8191SE wireless card in my Toshiba Satellite. When plugged into Ethernet I reach download speeds of over 30mbps, but on my wireless card I'm lucky to get over .5, as well as frequently having limited access (the yellow sign on my Internet connect). I've updated my drivers, and it seemed to do no good. My current router is a linksys WRT54G (I realize I won't be able to get the full 30mbps on a wireless g router, but it's what I have for now). My iPod and phone are both able to get around 12mbps off of the same config. Is there anything I can do?

Is there a setting to configure the wireless card to work in 802.11g mode only, which would be appropriate for a WRT54G router?