I'm not 100% sure but I think its like 2.11gigs for application data and 12.1 for other things such as music and pictures. Then the rest I have no idea.

As for rooting you can't change it, I don't even think you could with a custom ROM but yet again not 100% sure.

 

luckily most devs now are allowing apps2sd. Or if they dont allow app2sd, what theyre doing is putting the base 5 - 20 megs on the phone and making the install automatically put the rest of the info on the sdcard.

so no worries, it would take alot to fill up that 2 gigs for apps.

 

I put my old 16gb sdcard out of my Droid 1 in my Droid 3 and i dont think it really matters witch one you use, besides preference for organization. They're both sdcards, one is just built into the phone.