Definitely 2 monitors. I do alot of DB and SharePoint work and I have found the most helpful setup for me at work is to have one machine and one monitor in dev and have another setup in production with one monitor that way I can look at changes in dev and compre them when I apply in production or have to troubleshoot issues from one domain to the other.

I feel hadicapped a little when I only use one monitor now... 🙂