WiFi throughput is similar to that of a hubbed network in that it is a shared medium. For example: Say you have an 8 port 100 Mb hub, and you have 4 computers connected to that hub. What you have done is basically divided that 100 Mb bandwidth amongst the 4 computers, meaning that you have 25 Mbps per machine. Mind you, this is not a constant since not all machines are accessing data from the network/internetwork at the same time. However, you can test by connecting one machine on a hub and streaming music like through iTunes, and then doing it with all 4 machines connected and see if anything happens. Most likely, the more machines you have connected to the hub, the greater possibility of having collisions because of the nature of Ethernet.
WiFi is a little worse when it comes to speed. 802.11b is 11 Mbps and it's shared like an Ethernet connection on a hub. However, instead of being a bit variable as on wired ethernet, each machine reduces the throughput by half EVEN IF IT'S ONLY ASSOCIATED AND NOT ACCESSING DATA. Aside from that, your throughput reduces as you get farther away if your throughput is set to auto. You can force the bandwidth to be 11 Mbps for evey host, but it effectively reduces the distance you can travel.
802.11g is much faster and backwards compatible, but it's drawback can be said that it IS backwards compatible. If you have a bunch of machines using G and one machine comes in with a B connection (and this feature hasn't been disabled on the AP), ALL MACHINES ASSOCIATED drop down to 802.11b.
The moral here?? You connection is only as fast as your slowest link. Also, if you have the AP set to auto for bandwidth, you are at the mercy of the machine with the slowest connection. Another example: If you have all the machines at the radius for 11 Mbps, and another machine associates from a distance where only 2 Mbps is available, ALL MACHINES NO MATTER WHAT THEIR DISTANCE FROM THE ACCESS POINT are going to associate at 2 Mbps max.
Also, make sure that other devices that might interfere with the signal, such as microwave ovens and wireless phones in the same 2.4 Ghz spectrum, are not doing so. I have found out that channel 11 has given me the best signal when encountering interference from other devices, but YMMV.