When you connect "wirelessly" to the Router, the PC will show a speed of 54Mbps, which is the nominal speed of 802.11g wireless networking.
If you connect to the Router by ethernet cable, the data transfer speed will be 100Mbps.
Both of these are the speed at which the PC's port is communicating with the Router, i.e. the speed at which your "Local Area Network" is running. This is unrelated to the speed of your internet connection.
>>> But when i actually physically download something, i only download at around 400kb per second
I suspect you are downloading at 400kB per sec (kilobytes per sec) and not 400kb per sec (kilobits per sec), note the uppercase "B" which indicates "bytes" rather than "bits".
Your ISP's quoted speed (possibly something like "up to 8Mbps") is in "BITS", most downloads are displayed in "BYTES", so if you're downloading at 400kBps, that equates to 3,200kbps or 3.2 Megabits per sec.