The current version is 14, also known as CS4 when part of a suite. You cannot buy earlier versions from Adobe. It is only possible to buy older versions legally that have not been used as a basis for an upgrade.
8 and 10 should know work at least from 95 up to XP on a Win machine; I am not sure about Vista. I know nothing about the Mac OS versions.
On a Mac, AI8 will work in Classic mode up to at least 10.3.9 or maybe 10.4.11. AI10 will work in OSX up to 10.4.11. Do a search on the Apple site to confirm the last installation of OSX with Classic for a better idea.