There are some other improvements in the system architecture and early results show good improvements in system performance. While adding 4 more cores doesn't double the speed, these systems look signficantly faster than older Mac Pro's, especiallly in encoding video. Check out this benchmark test from MacWorld for more info:
"From the Lab: Internal changes bolster latest Mac Pro"
I was surprised at what low marks they gave the internal hard drive that ships with the new Mac. I've always assumed that those had decent performance, but MacWorld found a significant speed bump on their multi-app test by putting a hard drive from an older Mac Pro into the current 8-core 2.8 GHz Xeon. For some good specs on hard drive performance, check out Bare Feats. Apple also has some performance comparisons with other Mac towers here, including rendering & encoding with FCP and After Effects.
There had been rumors that Apple was ready to add a Blu-ray drive for playing back high-def movies to the Mac Pro's. For PC's, adding an HD DVD or Blu-ray drive, or both, is now a common option. Nothing from Apple about Blu-ray for these new computers, but an announcement could come at the MacWorld Conference. I'm anxious to see Apple add Blu-ray disc authoring to DVD Studio Pro, as well as decent support for the HD DVD standard. Adobe's Encore already does this. Commentator Bob Cringely suggests that Jobs will ignore high-def discs because he sees them as a threat to iTunes ruling the media world in his latest column (he also thinks Apple will buy Adobe, which seems a bit far-fetched to me.)