How do you improve an iMac G5 PowerMac8,2 wireless internet signal? Update a driver?

It is hardware issue not software, so updating the driver will not help. If you can, try relaying out the iMac in the room, sometime moving the machine to the other side of the room, or just turning it can help a lot. If not there are a variety of 3rd party products made especially for mac that fix this. They are UGLY, they usually take up a USB port (and should not be used through a hub) but they do work GREAT. See below for an example: