Well, I think it boils down to this: I believe the original poster wants to "tinker" with the iMac -- trying out different OSs, seeing what the computer is capable of... which is a legitimate task... I'm a tinkerer myself.
But the difference is the legality of going about that. You need to first determine what OSs the iMac is capable of running. I think we're in agreement that Mac OS X 10.0 through 10.4 are capable of running on that machine, even if the iMac needs a RAM upgrade and even if 10.4 needs to be acquired on CDs (Apple only ever offered a trade-in program to obtain a legitimate copy of 10.4 on CDs -- you had to first purchase the DVD of 10.4).
You're probably not going to be able to do this without spending money. You need to purchase a licensed copy of some version of Mac OS X that is compatible with that machine, or have a "friend" that is willing to simply give you (read: NOT copy their disks) their old Mac OS X install CD and relinquish all copies of said CD, so that the license is completely and legally transferred to you.
Once you have a legitimate copy of Mac OS X that is compatible with the iMac, we'd be more than happy to help you get the iMac running Mac OS X. Until then, though, I think you're just plain stuck -- you do not have a licensed, legitimate copy of Mac OS X yet, and your hardware may need upgrades in order to run certain versions of OS X.
As the convenience store down the street once said to me, "Come back when you have money."