First confirm if they will automatically change your order, they might, depending on configuation. Or they may have to contact you due to change in price or something.
Hard to know without some real hands on.
I know that DDR3 + Nehalem = huge improvement in memory bandwidth.
But I see where the new low end 4-core maxes out at 8GB RAM.
http://www.barefeats.com has some early "ideas" of what to expect.
Clock for clock, Nehalem is more efficient and can under heavy use run two threads per core.
Personally, I am surprised at the cpu configurations offered.
Some thoughts and info on ECC and DDR3:
http://forums.macrumors.com/showthread.php?p=7202196#post7202196
- with a link to review on Tom's Hardware
And their Mac Pro forum is buzzing, too!
http://forums.macrumors.com/forumdisplay.php?f=1