Definitely try recompressing to Photo-Jpeg.
I use, 640x480 clips Photo Jpeg 80% on MBP 2.5Ghz 2 Gb RAM. and an external firewire 800 HD. (7200 rpm) I really noticed changes in performance with a 7200 RPM HD thow FW800. Even thow is not as fast as a RAID, it works for me.
I also conected the HD throw esata adapter, but more headeaches than solutions, so I went back to FW800