If you have seriously fast harddrives it doesn't do that (not noticeably anyhow, and I'm talking RAID0 Caviar Black type fast)
There's also the option of Xfire, and FRAPS can have the settings turned down to be less intensive IIRC. It's been a while since I've done any of this though tbh
I know Steam recently added screenshots, not sure if they added any sort of video functionality at all though =/