Using a wireless bridge to pick up the wi-fi signal and then feeding it in through the ethernet port has seemed to be a much more stable (and quicker!) way to go than any of the USB dongles I tried... I know that some routers can be turned into wireless bridges (receivers) by using DD-WRT firmware in them... They become much more versatile that way!
The only problem I find is the limitation of the iMac to process all the junk that's added into today's web pages... That can gum things up so quickly.