Greetings, and welcome to the forums!
The output on the X6 Ultrabase is analogue but most monitors sold come with either analogue or digital input - some with both, others with either one. Your purchase of the monitor should not be limited to the connector type for the simple reason that there are conversion connectors readily and cheaply available that, if need be, convert the analogue signal for a digital connection. Thus, it really is up to you, the deals that you can get on an external monitor to make a purchase of such.
Next, there's no worry about running dual monitors on the X61. Using either Vista or XP, you'll be able to seemlessly use either monitor or both. You can make the selection in the display properties (of either operating system). No additional software required for this to work.
Hope that helps
