The bandwidth on a usb connection sucks for running an os off of. If I were you, I would crack open that enclosure, and physically install that drive in the machine on its internal bus.
Secondly, Yes, you can do dual monitors. The details vary based on your distro and what video card you use. It is VERY easy with a modern Nvidia card and the proprietary nvidia drivers. However, I have had very poor luck getting 3d games to run properly with both monitors running. Not hard to work around, you just need disable of on them when its game time.
Good luck, be prepared to ask lots of paitence, and spend lots of time here and on google.