so Nvidia has Shadowplay, and AMD has this: http://raptr.com/amd
I use it to record sweet janetside clips of me owning a bitch so hard he reports me to Sony for hacking and then I upload the "hack" kill to youtube just to make him cry, and then he responds maturely like a human, and then we become friends: https://www.youtube.com/watch?v=TTvEeTM5uIw
Anyways. It can record games, by using some magical hardware chip on your GPU, and the best way I can understand (and I'm not really sure this is correct, but its what I want to believe, dammit) is that it sits on the frame buffer, and writes frames to your HDD, so there is zero GPU/CPU overhead, as its just writing frames to disk. The only overhead is I/O, but if your running your shit off SSD which you should, and your OS is not pegging the swap file (which it shouldn't be, else you need to fix that shit), than you literally won't notice.