Hi,
Firstly regards the database extension, here is a link that will help you out;
http://www.msexchange.org/ExchangeNews/September-2005-Exchange-SP2-Database-Size.html
Regards the DC issues, I am not entirely sure. I have seen Exchange have the sort of problem you are seeing, you seem to suggest that after a few minutes (of the first DC going down), Exchange fails over to the other one. That I believe is usual.
What are you doing with nslookup exactly? It wouldn't normally fail over to a second server if the first one fails.
Cheers and sorry for the delayed response.
Nathan
Nathan Winters - MVP Exchange Server
MCSE & MCSA 2000 & 2003 + Messaging, MCITP Exchange 2007, MCP, VMWare VCP v2 & v3.
Welcome to the Microsoft Messaging and Mobility User Group: http://www.mmmug.co.uk