Will iMac G5 recognize external cd/dvd rom?
You don;t say what OS version you are using on your G5, but if you use a standard non-fancy external USB attached DVD Rom drive, then I can't see any issues that would stop it from reading the drive an an OS level, but I'm not sure that this will be the case at the firmware level.  So you need to factor in how you will perform the OS install, and is this just a newer version of OS X (in which case you are probably good to go) or another OS entirely, in which case I can't say...