if its a slot loading imac, then it will support the current os x, 10.4/tiger. but if it doesn't have a dvd drive, then you'll have to order the cd version of 10.4. otherwise the latest os x you can run is 10.3/panther. just remember, apples says that it will run with only 256mb ram, but it'll be really slow. anything under 512mb ram is sluggish, and 1gig ram is comforible. then the os alone takes 3-5gigs hard drive space.