The easiest way to test them is probably to leave them running something intensive like SETI@home for 24 hours. If they are still chugging along happily after that then it's a stable overclock. The only extra cooling I've ever used it is apply some thermal paste and put in a PCI slot cooling fan. The G3's don't run very hot so overclocking won't turn them into a toaster necessitating liquid nitrogen [] ]'>