KVM's and Synergy
Mark Watson writes about using a KVM switch to share screens between monitors.
Even better, if you have the desk space, is Synergy which allows the mouse to move between machines (Windows/Linux/Mac). You can even copy text between OS's!
This way, there is no context switch... you can keep reference/email up on different machines. Most folks have spare screens as the move to flatscreens continues.. why not use them?