It’s not that simple, though. A weak signal, in the absense of interference, can transmit just as much information as a strong one. The problem occurs because a weaker signal is easier to “lose” in the background noise, meaning dropped packets, etc.
Furthermore, you never get the full bandwidth of a connection – 100baseT might only give you 80mbps even with only two machines directly connected. Collisions, NIC speed, cpu speed, traffic shape/type, and a host of other factors all have an effect on overall bandwidth.
It is likely that the wall attenuates the signal a bit, but it also might be that the downstairs neighbor has a cordless phone under your room, or they use the microwave, or there is RF leakage from your TV, or your laptop wireless antenna is in a noisy spot, or something else.
Like others have said, move your laptop or whatever and see if the wall makes the sole difference, etc.
Also, depending on your wireless router, there is alternative firmware like Tomato that allows you to set your signal strength, so you can boost the gain on your router by about 250%.