Hi Hugo,
I'm not an expert or seasoned user of VW, but from my limited experience, it's extremely fast. Now, comparing an iBook or G4 to a dual G5 is not fair. The dual processors in a G5 are well used in intensive rendering etc. Others with more experience can chime in also.
BTW, I bought my G5 1.8 dual computer and 20" display in Dec. Display was 1,300, now it's 800. CPU is now 2.0Ghz and updated at the same price 1999. If you go to a 2.7ghz cpu, you'll scream through your renderings.
Take care,
John