dns server sizing?

[Michael Cunningham]

Folks,

I am currently redesigning the entire dns environment for
a medium size company with about 10k employees. I am trying to figure
out how I should best size the dns servers we will need. We are
either going to go with solaris on sun boxes or linux on pc's depending
on what managment wants to spend. This company has a world wide 
network in dozens of countries around the globe. We will be setting
up one internal master dns server for the whole shebang and a whole
bunch of slaves, Probably 10 or 12 around the globe on the wan. 
The internal dns will forward all non internal lookups to a master
and two slaves that are outside our firewall which will know about
external dmz address and be able to do internet lookups. 
The external dns is going to be hit hard by the net and our external mail
servers which transmit about 200k emails a day. Can anyone give me 
any type of sizing guidelines for dns performance? I assume cpu and
memory are essential for decent performance? Suggestions on what
I should pick up for the 2 masters and the slaves? Pc based
recommendation? Sun based recommendation?

Thanks for any info you can offer..

Mike