Well, your going for too much for that game there

Let me help you out a bit. Firstly, I don't own City of Heroes but City of Villains instead. The minimum requirements are as follows:
Intel Pentium 3 800 mhz, 512 mb of ram, 4 gb hdd space, ATI Radeon 8500, 16-bit sound card, 56k modem, directX 9.0, and a keyboard and mouse.
Hopefully you have all those

Anyways, that mac will come with ALOT more then you need. The recommended settings for what you (want) are as follows:
Intel pentium 4 2.0 ghz (iMac 24" pro comes with 2.8 Intel Core 2 Extreme processor Way better than needed), 1 gb of ram, 4gb hdd space, About 128 mb of vram, 16-bit sound card, Broadand Internet connection, directX 9.0 and a keyboard and mouse.
Basically, if you get that iMac, you'll be at max settings enjoying the game to its fullest. The only problem i can see is maybe the game isn't out for Macs? I have mine for my old windows pc i used to play some of my older games.
ALWAYS check the bottom of the box for the OS though. I almost made this mistake and almost ended up wasting 60 bucks. Of course, they would probably have let me return it, but i almost bought a game for Windows. As i said, ALWAYS check the bottom of the box for the OS. You should see something like Mac OS X 10- then system requirements.
A game called World of Warcraft as long as eiter many or all of the Blizzard games are made for both Windows and Macs. Be sure to read the system requirements even if it says windows, because at the bottom there may be another section for the Mac OS X. Anyways, good luck and you'll be more then happy with that iMac as it will run anything.