Windows

You would create a second Active Directory site in the administrative console Active Directory Sites and Services.

You would then associate the subnet with that site.

At this point, systems registering in DNS as being in that subnet would be be identified by DNS records as being in that AD site.

How many clients will be in this other building?

What applications will they be running?

This will determine not only your bandwidth utilization, but if you require a second domain controller in the facility.

You will not be able to block traffic between the two networks or you are going to have major issues with systems falling out of Active Directory.

Thanks
I will be having 64 clients in the 2nd building; I have already created the site and assigned it the subnet (a range of 100)
Around 50 of them are just using a web browser to access a virtual call centre package and possibly VOIP to a telephone switch located in our current office (Not decided on yet), while the rest (Managers) will be using email (Exchange) printing and file sharing. Hence the new print and file server

I am a little unsure on:
?At this point, systems registering in DNS as being in that subnet would be identified by DNS records as being in that AD site?
Obviously when I add new XP machines to the domain, I let it obtain an address by DHCP automatically. What stops new machines in the 2nd site just getting an IP address as part of the main site?

humm, are in the previous question 50 ft distance isn't that far. The nebulous aspect to all this is bandwidth utilization which until someone actually starts monitoring the link to determine just how much bandwidth is being used [including overhead] the usage remains an unknown. That unknown is the crux of the problem. Can start limiting traffic until you know what to limit and you don't know what to limit because you don't know whats eating up bandwidth. Could be DNS, could be DHCP but more likely, clients accessing services on the network and any Internet access will eat up the most.

As far as clients in the second building obtaining an IP address from DHCP running in the first building, as long as there isn't a router between the 2, DHCP discover, offer, acknowledge traffic will reach those clients. If you stick a router there, you'll have to install a DHCP relay agent because DHCP packets dont get fowarded by the router.