Obviously you'll never get to 600Mhz, try for 400Mhz. G4s don't tend to overclock as well as G3s. Generally the CPUs run fairly cool but a quiet fan screwed on top with better thermal paste would be a must. My experience with overclocking G4s is that they either work 100% from the outset or die very quickly with Open Firmware dumps or kernel panics. It's not like PC x86 overclocking where it takes a lot of tweaking to get things running stably.
JB