I never did do any speed tests, but was a very big difference.
I used vmware, virtualpc2007 is free now, or can run the live cd's directly each computer.
What makes this great is that your cpu should never hit 100%, it will take from the "pool", and you should not run out of memory as easily.
40 dual cores and 80 gig physical memory, that would be pretty good I'd say.
You will still have to deduct any usage from the main os and clusterknoppix, but it didn't matter to me much when you have access to the "pool"