"ip helper-address x.x.x.x" is all you need on the branch office router interface. The statement converts the broadcast into a directed request to the target specified.
If you have multiple VLAN's in the regional offices, each requiring DHCP services, you'll need to add "ip helper-address" statements to each of the VLAN sub-interfaces on the router.
Of course, you'll need to configure the DHCP server appropriately. It'll know which DHCP lease to serve the IP addresses from.
You can also use multiple "ip helper-address" statements too, for multiple DHCP servers. However, you'll need to carve up the DHCP lease pools, so that the leases don't clash.
One problem we've experienced - PC's may not be able to talk across the LAN segment if the WAN link dies, so you may want to have a think about how you're going to serve out IP addresses during WAN failures.
