I see this post is very old, the OP probably is long gone, but in case anyone is interested, now the post has been raised form the dead by krfm.
If you have two medium sized networks then pay a profession to set up and maintain a solution. Think about how downtime will affect you if you screw it up yourself.
If you have one network server set up and just need to connect a couple of pc's to it from another location, create secure VPN tunnelling to it from each desktop machine (free), but then again do you know how to set up MS Exchange Server on your network for this?
I think the OP needs cloud computing to hit the mainstream
Alternatively if it's only data that you want to share between the networks consider cheap web-based data storage (for God's sake always KEEP AN OFFLINE BACKUP TOO....)
Read some info at:
http://www.pcworld.com/article/125729/s ... e_web.html
This is fine unless you need to have two or more people working on the same data (a shared spreadsheet or whatever) at the same time. You should ask the storage provider if their service allows this.