I have a fairly complicated wireless set up in my home. My device of choice has been a Linksys WRT-54GL loaded with dd-wrt. Recently I noticed that my transfers over wireless were running about half the speed they should be. I was actually at the point where my Charter Cable connection to the internet was faster than my wireless connection. After verifying that the problem existed for multiple laptops I started trying to track down the cause. Switching wireless channels, simplifying things a bit, going back to WPA from WPA2, switching to G only. By the way the G only broke my WRT-54GL that acts as a client for the TV computer downstairs. None of the changes fixed the problem.
I finally ran across a post on the DD-WRT forums that talked about a similar problem. Their solution was to restore to factory defaults and set the router up again. This solved the slowdown problem! It also prevented the downstairs WRT-54GL from working with the newly factory defaulted router upstairs. To solve that I had to reset it to factory defaults and set it up again. When all was said and done I'm back to transfer rates around 2 Megabytes per sec. Which is about the best I ever get out of wireless G.