How to setup multiple DHCP ranges for a single office

You would only have one DHCP server, just setup multiple DHCP scopes. Information on configuring scopes here: http://technet.microsoft.com/en-us/library/dd759218.aspx

We're doing this with Server 2003 where I am. The key is our central layer 3 device (in our case an old 3Com 4900 SX). It has what 3Com calls a "virtual interface" for each vlan. When dhcp requests arrive at the device from the outside it forwards them using same vlan interface they arrived on. The dhcp server just has a scope setup for each vlan with no other special configuration details. It matches the dhcp request to the originating interface on the layer 3 device to know which scope to use. That's out of the box behavior for the windows dhcp server.

The downside to this approach is that segment with your server must now be a member of all vlans (on the same virtual segment). This means all your broadcast traffic will now go up the fiber link to your main office (though not down all the other links to other branches), and it's nice to keep those links as clean as possible. It'll also use time on what is likely already your busiest local segment to reach your server. But this is probably unavoidable anyway.

Update:
Looking again, we do use the 003 (Router) option under Scope Options to match the address of the virtual interface with the correct vlan. That may have something to do with it as well, but I think that really just sets the default gateway.