If I understand Post No. 7 [by you] correctly, then you are getting 51 Mbps download while connected via Ethernet and usually 15-18 Mbps upload when connected wirelessly. chown33's link says that the maximum real world throughput of Wireless-G is 19 Mbps. This is just slightly more than you are getting.
This is exactly right.
54Mbit 802.11g on 2.4Ghz will yield about 20Mbit
100Mbit ethernet will yield about 80-90Mbit
10Mbit ethernet will yield about 8Mbit
This is all dependent on protocol used and packet overhead and the speed of everything between point A and point B. That includes the speed of the processor in the router etc.
Also routers and switches with 'Jumbo Frames' will be faster as they support larger packet sizes. The backplane of most SOHO gigabit switches it not sufficient for gigabit on every port, same with 10/100. This means if you have traffic on every machine, it's shared bandwidth. Commercial routers and switches can handle the traffic so that is not an issue.
I had similar problems at my SOHO so I invested in a good Cisco router and Cisco switch via E-Bay as you couldn't afford them in your home otherwise.
Basic rule of thumb for networking, if you are paying less than $10/Mbit on a switch/router, you will not get full speed and usually not close to it. This isn't a statement saying to go buy expensive gear, it's a note that us poor people can't afford full speed and we just need to be aware of it. That said you can get some decent used hardware if you are diligent enough.