Correction:
"... Most PowerMac G4 and newer Macs - these all have auto MDX ports."
Auto MDX is only a standard feature on 1000baseT (gigabit) Ethernet. *Some* 10/100 Ethernet cards and switches implement auto MDX but not by any stretch all of them. (Most new 10/100 switches do but if you're looking at a several-year-old one chances are good that it doesn't.) The reason you wouldn't under most circumstances need a crossover for a switch or a router's access ports regardless of speed is all but the uplink port are likely to already be wired as standard access ports.
I know, nitpicky.
The other thing to watch out for: Sometimes those old ethernet transceivers can be picky about connecting to any switch/ethernet port that tries to autonegotiate anything faster than 10mbit. There have been times dealing with elderly equipment that I've been forced to stick an old 10mbit hub between the newer switch and the computer to get things working.