one core, two core, four core, eight core... show me the software that will run on it and I will buy! As to the question about having a lighter tower....does it really matter? If you want a portable buy a macbook pro. I normally put it under my desk and that's it... the butler moves it to clean not me!
Modo recognized 16 cores in it current version, Zbrush 3 will see up to 256 cores.. i expect all the Apple pro apps to make use of 8 cores. OSX 10.5 will.
I would max out those 8 cores rendering images, and encoding video. Though i do think until more basic software is able to see more than 2 threads on avg 4X 3ghz cores would be better than 8x2.33 ones .. it depends what u do..