Well, no offence intended, but your comments are at odds with what certain others have said. Maybe you're right, but I'm not gonna drop that kinda money when there's any doubt about it!
No offense taken. I'm not sure what others have said but I can only speak from my own personal experiences. My i7 plays the games I enjoy very well. Do I expect it to run the games of 2 years from now at native resolution with blindingly fast frame rates? No.
Perhaps a home-built PC is your best solution. That way you can put in exactly what you want and have the ability to swap out parts in future.
I do understand your complaint about the relatively dated GPUs Apple uses in their machines, so we're sort of going in circles there. Yes, if it is critical for you to get a machine with the very latest GPU the iMac is not for you.
What is it that attracted you to the Mac anyway?