2000 to 2008 R2 Migration and Active Directory Database Size Growth

I have 6 2000 DCs and I am planning to migrate to 6 2008 R2 DCs on new hardware.   I plan to introduce my first 2008 R2 DC in the next couple of days.  As you can imagine, our new hardware is wicked fast compared to the old and the new storage is about 40 GB per db/logs/sysvol volumes.

Our current average database size is about 700 MB.  I understand that the 2008 R2 AD database increases by a considerable amount to accomodate changes such as no more partial merge, two new indices on the large link table and if using the AD recycle bin.

This Microsoft Assess Hardware Requirements article leads me to believe our post-migration AD DB size will be 2.8 or roughly 3 GB based on 7500 users.

Here is my problem:  

The article tells me why the size will increase, but it does not tell me *when*.  

This scares the hell out of me as I am afraid that when I bring the first 2008 R2 DC online, our AD DBs will grow to 3 GB on all of the 6 2000 DCs immediately.  

On each 2000 DC volume that currently holds the AD DBs,  is limited space (by today's standards)...

DC1 - 9.3 GB
DC2 - 9.3 GB
DC3 - 5.3 GB
DC4 - 5.3 GB
DC5 - 5.3 GB
DC6 - 9.3 GB

I worry that when I dcpromo my first 2008 R2 member server to a DC, it will negatively impact my production environment.  I could really use an online resource that answers the question - when will my AD grow?  Thank you.