You need an insanely fast system to do on-the-fly recording during gameplay without it affecting framerate, I'd say an FX-60 writing to RAID0 raptors at the bare minimum. You also need to turn the resolution right down.
A far superior option is to record a standard in-game demo while playing, and then take video capture from demo playback. This is how all the 'pro' gaming movies are made and means that you don't have to cope with your harddisk being written to 30 times a second and the subsequent lag which accompanies that.