I did not realize that iMovie would capture game footage or was it captured with something else?
Back in the days when I was a full time Windows user and gamer I purchased a neat app called FraPS (which is a benchmarking tool for Frames Per Second (FraPS, fraps...) available from www.fraps.com - When I installed XP on my MacBook Pro I just downloaded it so that the life fps would be displayed in the top right corner. I just press F9 and it'll record the screen for me as an uncompressed AVI with audio.
When I'd got 15 minutes worth of footage I just rebooted into OSX and mounted the XP partition, nabbed the .avi's from it and after converting them to something Mac-friendly in QuickTime I dumped all of the clips in iMovie and chopped 'em around until I got something of a reasonable length to play with.