You can probably get an imac G4 in the $550-650 range on Craigslist or ebay. Otherwise, if you just want to try out Mac/OSX, you can get a G3 imac for $50-150 on either site. Play with OSX, just realize that on any other modern Mac, it will run much, much faster. 😉
I'd consider a G4 tower, though. You can probably find a low end one in your price range right now that will come with a keyboard and mouse (again, Craigslist and ebay). And check your local Craigslist for monitiors; CRTs are being given away and left on roadsides in Portland, my company got 5 free from a company downtown that was upgrading to LCD, all 5 were 17 inch flat screen projection monitors, and 3 of them were pristeen (2 had some burned images into the tubes, but nothing we couldn't use). I'd say that, for maybe $50 more than what you've got, you can be up and running, and the G4 is heavily upgradeable, so you can keep improving it as you can afford it, or save up for an intel mac later.