Re: Active Directory Sizer for Win2k3 DC?
You might want to take a look at AD Sizer (http://www.petri.com/active_directory_sizer_tool.htm), which is a bit outdated, but will give you a good clue about the HW requirements; and SPA (http://www.windowsnetworking.com/articles_tutorials/Diagnosing-Server-Performance-Problems-Server-Performance-Advisor.html)
All that said, 300K users is a LOT for only 2 DCs. Are you sure you have not missed couple zeros ?
For 300K user distributed environment I would expect something around 100-400 DCs (if you are world-wide) and several dozens DCs if you are centralized.