OK, the easy bit first. The 54 Mbps refers to the maximum available speed between the wireless devices. In fact you?ll never see this because of overheads and the like. But basically this is the connection speed between WiFi devices and the WiFi access point (AP).
Depending on the contract that you have with your provider you?ll have a service with two speeds defined. ADSL supports maximum data rates up to 40 Mbps when receiving data (known as the downstream rate) and from 16 to 640 Kbps (this may be slightly higher now) when sending data (known as the upstream rate).
I?m not sure what you tested to arrive at 400Kbps, but this is very respectable if you have a downstream rate of 512Kbps.
For the 2.4 GHz wireless LANs (which most systems are), there are many sources of interfering signals. For example microwave ovens, mobile phones, wireless phones, Bluetooth devices, video senders, cars etc etc. Don?t forget that these signals can be generated by your neighbours and you won?t be aware until you start to study the problem.
One suggestion is that where you have your desktop, downstairs, that area is being exposed to interfering signals. You should only consider this a problem is it?s affecting your throughput or dropping you off the network. If you?re ?happy? with the throughput of the desktop computer, do nothing ? live with what you have.
If not and if you cannot remove the source of these interfering signals try changing the channel that your wireless AP is using. Generally these systems are configured to automatically select the best channel, but they don?t always get it right. You?ll probably have to try this a couple of times using channels at either end of the range. You may also need to change the setup on the computers so that they match the channel number on the AP.
Good luck.
