Very low speeds over 54g wireless lan

I have a belkin 54g wireless router and a lynksys 54g wireless adaptor on a pc. a second pc is on a wired connection to the router. My problem is this, while internet operation is comparable on both machines, via a 2Mbps WAN, when i copy files from one pc to the other i am only achieving speeds averaging 5 or 6 Mbps. Until i did some reading i was under the impression i would achieve a transfer rate nearing 50Mbps over the wireless link but now know only 24Mbps speeds are typical, i don't know why... Anyway, it took 20 mins to send 900MB and i've calculated this to an average 6Mbps (if i did my sums right!). Why am i experiencing such abysmally low transfers, and what can i do about it?

Incidentally, there is a laptop on the wireless network also, it can only achieve speeds of up to 11Mbps, i believe it's a 802.11b adapter onboard. I read that the 802.11g protocols include compatibility with 802.11b but also that the wireless lan will only operate at 802.11b speeds where there are 802.11b clients. However, the laptop is not involved in this scenario, transfer is between a wired desktop and a 54g desktop via 54g router.

Hope someone can help because it's almost too slow to be useful, thanks