Final Clarification on using the iMac as a monitor

Hey guys, hoping you can help.

Right basically i have been looking at getting a GFX card with the mini-display port on so i can hook this straight up to my iMac.

I just need some clarification on if this will defiantly work with money being tight and i don't want to waste my money.

btw does anyone do this?

The parts I'm ordering will be:

The belkin Mini-display port to Display port cable

Also whoever has done this, is it worth having it on the iMac 27inch for gaming? thanks in advance.

GFX Card to go in the PC with display port

It would help if you told us which year model your iMac 27" is? The 2011 iMacs are equipped with Thunderbolt outputs whereas earlier models are not.