SBS and Win2k3 on Same Domain

I have a server running SBS 2003 and I need to purchase more licenses for my users. However I plan to add a server at 2 other locations as well which would be running win2k3 Standard.

Question 1: Can they exist on the same domain? If not I will install win2k3 on the server I have now running SBS and go from there. I am in the early stages so I am not losing anything.

Question 2: In terms of growth, should I be purchasing more SBS liceses or purchase win2k3 standard licenses?