router-to-router questions

Hello,
Sorry if this questions are stupid, but I'm just beginner.
I'm going to implement router-to-router two-way communication between two
our offices using VPN. Right now I have the following structure:

[Office A, company.local domain with 1 DC (W2K3, DNS), member-server (W2K3,
SUS, RRAS, WINS)] - Lynksys router (DHCP, 192.168.20.x) - cable high
speed - Internet - cable high speed - DLink router (DHCP, 192.168.25.x) -
[Office B, workgroup, W2K Pro computers].

Both routers get external IP from ISP, but we use DDNS service for office A,
so VPN clients have access to VPN server in office A without any problems.
For router-to-router connection I'm going to bring W2K3 server to office B,
setup RRAS, DNS, WINS, subscribe for DDNS and so on.
I have a few questions:
1. I'm going to install AD in office B. Can use the same domain name or it's
better to build separate domain and trust relationship between two domains?
2. The question about network traffic. How much it increase the traffic in
the networks? For example, when workers in both offices use Internet (not
VPN) how the Internet traffic goes? I'm worry that it will significantly
slow down our networks.
Thank you so much for any help, I really appreciate it.
LB.