Do you want to record your XBox gameplay on the XBox or on the iMac?
Recording with Hauppauge HD PVR
Elgato Video Capture device
In the end, you need a capture card connected via USB or if you have a miniDV camcorder with video input, you could loop the video through the camcorder or record to it.
Then use iMovie to edit the footage.
Btw, your signature uses the capital "I" for the i-devices, though Apple uses a lower case "i". Examples: iMac, iPod, iPhone, iPad