Will Mincraft work really well on an Apple 27 iMac 3.2GHz?

I believe so but you may want to try the demo to be 100% sure.

This is hard to tell unless we have more information about your computer.
Gaming performance depends on the GPU (graphics processing unit) in the computer.
This can be either in the form of a discrete GPU (a separate card, which iMac computers generally do not have) or an APU (combined CPU and GPU).
I can just about guarantee you that your computer can run Minecraft, as almost any system is capable of running it. Minecraft is not a very graphically intensive game, even at high settings.
TL;DR unless your computer is broken, yes.