Couple of ways this is possible,
1) Use VNC to remotely access the Mini from the iMac, ( not sure if this would work well with games/3d graphics thou...
2)Use something like a EyeTV hybrid as a video input source, you would need to run eyeTV all the time on the iMac, but it would allow you output the Mini's video to the iMac in real time ( although the best resolution you will get is S-video quality)