It has been requested by the dev that is working on a launcher for OSX to poll for the flavors of OSX the population is most commonly used.
xcode doesn't even let me compile for 10.6 so os9 is totally out of the question send the hate mail to apple
PC client 100% doesnt matter what they play on......since the client doesnt impact the OS like on a mac.
I'd say keeping up with the most recent version and Snow Leopard (10.6) would probably support the vast majority. As mentioned, 10.6 was the last release to support Core Solo and Core Duo CPUs (think white/black MacBooks, and early versions of MBPro and silver iMacs).