WAN setup

We have our HQ at one location with 20 users (XP & Vista) on a SBS2008 domain.

We have just taken over a satellite office of 13 users (xp & vista) working off a XP Fileserver (workgroup) and wish to intergrate these users into our existing domain.

So i was wondering what methods others were using/recommend before we go about implementing a WAN network.

for example: As we have an Untangle server at HQ should we look at installing another untangle server at the 2nd location and using site to site vpn?
Do we need to change this workgroup over to our domain with the same subnet but different ip range?

Many thanks in advance for any recommendations.