Installing ram and getting 256-bit access

I understand that when installing ram in the new 2008 mac pro. You can achieve a slight increase in speed. By installing your modules in sets of four, of the same size.

Does this mean that once I have my first set of four. That all additional sets of for have to be the same type of the first. Or can I mix and match sets of four.

Will 4 1-gig chips, and a second set consisting of 4 2-gig chips. Still allow me to take advantage of the speed increase of 256-bit access.