there is no way to put snow leopard on a imac g3 and dont even think about leopard
it runs on tiger or panther , and is then still usable , no need for snow leopard or leopard ,
if you have upgraded the ram , fit in a 7200 rpm harddrive 120gb (128gb is the most it supports internal)and
you will see with panther/tiger on it its really usable, a bit faster on panther , but more programs run on tiger , and it will boot quite quick and the speakers are better then in the new 27" iMac , so use it as a itunes server, web browsing ,office tasks , if its one with dvd fitted you could watch dvd's on it in your bedroom (its nearly silent running as no fans inside which makes it perfect for a bedroom )
only because you cant install snow leopard/leopard it does not mean a system is obsolete and useless