Though based on real numbers, we did not test each individual machine, but relied instead on what we know about the performance of G4 upgrade cards, and the relative performance of each Macintosh model. In deriving these numbers we tried to be conservative in our estimates, so that, if you are surprised, you will be pleasantly so. At the moment dual processor aware applications are even fewer. This should change once OS X is released and more OS X savvy applications come on market. We do not, as of yet, have any performance scores for what the G4 in dual processor configuration will do in terms of performance improvement, but hope to add those scores soon.
Higher scores mean better performance.
If only certain parts can be accelerated what is this limited to. However, this assumes your code can be rewritten to take advantage of AltiVec. Many things can't. What deveopers generally find is they can accelerate the part of their code that does these things anywhere from 1.5 to 16x. But there's a lot of 'overhead' code that can't be accelerated, so the final acceleration ends up being substantially less.