For Macs, Flash is one of those iffy situations that depend on CPU power. An older iMac with a G3 CPU might not be the best idea. Also, most of those machines need to run OS9 instead of OSX because of hardware restrictions, but they might be able to run 10.2 or 10.3 with a ram upgrade. I would think an eMac or 'basketball/lampshade' iMac, both having G4 CPUs, would be the best option.