Fraps.
My rig is decent but not top of the line anymore. Some AMD 3,4 dual core and a Geforce8800GTS or something. I still get 30 fps videos in high resolution (1280x720) with full details. I haven't noticed the game slowing down too much, although I get a very noticably jerk when fraps starts another 4gb video file.
If you play in even higher resolutions then tell fraps to record in half resolution ... otherwise your system is very likely to stutter, which will reflect negatively on your video AND your game. 
And you need LOTS of hd space since fraps doesn't compress videos ... raw footage only.
An alternative to fraps is GameCam, although I'm told that the demo version doesn't allow higher resolutions. Fraps limits video length to 30 seconds.
Can't say which one is better if you decide to buy one or the other version since i'm happy enough with my fraps.