Vegas will run on just about anything, particularly when it comes to editing DV. The Dell Precision 390 will work just fine, although it will have to be ordered with the optional IEEE-1394 "Firewire" card. An extra data drive in addition to the system drive would be a good idea.
I have two that are used every day in a school video editing suite and they are rock solid. 4gb of memory, three HD (1.5TB) and dual core processors. And yes, the dell firewire cards work just fine.