Yeah - I agree with AviD - you should be able to AT LEAST pull 80 - 100 fps out of that rig NO PROBLEM.
I also agree that the software is probably the culprit. Maybe you should post your scores so the dudes can have a look. Once busy, Avid, Foo, etc. sees your scores they can probably give you an idea on what you will need to tweak.
Good luck dude!
Which reminds me why I want to start working on a beowulf-on-a-backplane design: I want to be the one to write the program called "clusterfsck."
-- Adam J. Thornton, a.s.r.