Use an iMac as display for my Windows PC

I currently own a MacBook Pro a Mac Mini and a Windows PC. My whife want´s us to get a new cool iMac but I can´t get rid of my Windows PC because of my gaming. I saw somewhere that you could use your iMac as a monitor for other computers so I started looking around. The thing I notices is that the current iMacs only have Thunderbolt and no mini Displayport. It seems that if I wan´t this to work I will have to buy an older iMac with the Mini Displayport. Do you guys know if this will work at all and if it works with all iMacs that have Mini Displayports or just the 2009 models for example. The graphic card I´m using in my gaming rig is an HD7970 with Mini Displayports.

This is my first post here at Apple Support Communities and english is not my native language so just let me know if I should clearify something or something.

Best regards

Andreas (Sweden)