Right now, I'm doing the stream (mjpg-streamer) of 4 cameras logitech quickcam pro 9000 at 960x720 @ 15fps, running on a single rpi model b, without problems.
The recording of the video is made through motion (kmotion) running in a diferent pc (intel D525MW, 4 Gb, 1 Tb).
Best regards,
MIguel