All of the wireless LAN technologies quote the signalling rate. The maximum actual throughput is half the quoted rate.

So with 802.11b you're looking to get about 4-5mbps and 802.11a/g is around 25. Some manufacturers implement their own systems and double the standard rates.

Overhead is greatest on 802.11b then USB and finally Ethernet has the lowest. You'll never get the full bandwidth because the protocol being used sucks up some of it.