I guess it will run, but it won't look pretty.
Is that a 27" iMac? Because you won't be able to run it at 2560x1440.
If you want to get back into PC gaming, you could use your iMac as an external monitor for a Windows PC. Kind of defeats the purpose of having an all-in-one, but you can use your iMac for work, surfing the web, etc. and switch between your Mac and your PC with the press of a button.
Still my personal dream... if I could afford a 27" iMac