Hi. I'm a bit newish to SI myself so I won't comment on that but my background is in IT and PC support so I'll shoot for the hardware side:
If you're using these PCs for business I'd hold off on Vista unless there are specific features you find compelling (and no Aero doesn't count

).
Vista, w/ Aero especially, is a memory and resource hog and you may find that it nullifies your gain in new hardware (e.g. it's no faster than your old PC would be). As always YMMV.
As for using your laptop on the road, you might be needing to go to a Lansync type setup in D-tools, and have a separate license for each machine. Talk to D-tools about that.
If you're not using d-tools extensively in the field you might consider a remote access solution for the desktop (something like a VNC or GoToMyPC type solution) and just remote the desktop when you're on the road. Depending on your desire for security, you may or may not justify installing a VPN and the overhead that goes w/ it for that.
In the end there are a number of ways to skin this cat.
Feel free to PM me or run this thread if you need/want more info.
J.