Gawd...
I reckon your Mac times would drop /signficantly/ (if a G4/500 takes 5 hours it might take 20 now) while the VPC unit would probably take a good 20-30 hours.
Just guesstimates, really. Bottom line is it's not worth it. You have to remember that since you're running VPC, you're already using up gobs of processing power just to do that, so it's not like you're crunching 2 units for the price of 1...