How to tell if it is Wireless-G or Wireless-N that the iMac is using? I sometimes see the iMac copying files from the PC wirelessly at a rate of 1.5MB/s and sometimes 2.5MB/s (by visually examining, because Mac doesn't seem to show what the speed is in the copy dialog).

Is there a way to find out whether my iMac is connected to the wired PC through the wireless router using Wireless-G or Wireless-N?  Can some app show and/or the terminal app show?
Option + click your menu bar's status icon.