One fast 7200rpm drive should be enough speed for FRAPS, as long as you are not using lossless RGB, or trying to record at much more than 1920x1080, 30fps. More than this and you would benefit from a RAID 0. If you go RAID 0, even a pair of modern 5400/5900 rpm drives would be more than enough.
I currently have three 7K1000.Cs in RAID 0 (128k stripe and cluster size, using a 2TB exFAT partition aligned/offset 384k) serving as my video recording array. Sequential write speeds are about 400MB/s, which is way more than FRAPS needs, but if I record completely uncompressed video with PlayClaw, the speed becomes necessary.
The most important thing is to make sure that nothing is using the FRAPS drive, or array, at the same time you are recording.