When I buy Fraps + an SSD, how big should it be? I'll be recording to that one. However, I'll also be transferring the clips to a 3TB HDD time to time.
It depends how much data you are capturing. IMHO, you should be capturing all the data to the external HDD. An SSD is not normally meant for storage (YET).
Use your SSD for boot and OS and other apps and your external HDD for storage since you have so much more of it.