I've used Snapz Pro 2 to capture demorec game footage from America's Army on my Dual 2.0 G5 with 2.5 Gigs of ram. It was at 800x600, millions of colours with sound and around 20 minutes of straight footage at 30FPS. It worked fine for about the first 15 minutes, but the last 2 got really choppy while watching the game recording play back. I don't think that it had anything to do with Snapz because it was the game that was lagging. Maybe it was the poor playback engine, or the combination of using two high-intesity applications at the same time while swapping huge amounts of data to the drive.
I haven't used it extensively so I don't have much more info than that. If you have the machine for it Snapz will be able to record your game footage, and I believe it is the only one out there for Mac that can do it.