"For most the answer is not to dedicate, unless you can't control application access to the DCs or you have another valid business reason. "
This is a little disconcerting because when I attended an Exchange 2007 configuration boot camp the recommendation was that yes, I should have a separate A.D. site. This, even though we only have one Exchange site, two hub servers and two pairs of CCR mailbox servers.
Also the 1:4 ratio on processors doesn't detail whether it includes CAS servers which the article indicates uses AD heavily. if I have 5 quad processor CAS, 2 quad processor HUBS and two pair of 8-core CCR mailboxes that is over 40 Cores requiring 5 8-core GC's.
Can this be?