I’m not a super-networking expert yet, but I know a few things. I work at an integrator that uses only cisco equipment and we would set up either a pair of PIX 501s or a ASA5510 at the larger office and a pix501 or 506 at the smaller office perhaps.
While you can set QOS on the network devices you control, you can’t on the backbone carrier and you may find the voice quality still has occassional issues. Hopefully not, but perhaps.
You wouldn’t be able to put all of the computers on the same subnet of course, but they could join your windows domain and share resources. Just make sure that you assign proper DHCP on the 6 computer site so that they ONLY have DNS of the ONE server you have. Otherwise you’ll have issues.
You could set up the VPN tunnel between the two cisco devices and tunnel only lan traffic and let other traffic you out each respective router directly to the internet.
For a true failover scenario it could be complicated and expensive because of the required hardware.
I would set it up this way:
1) Get a pix 501/506 for the smaller office.
2) Get a Pix 501/506 or an ASA5510 for the larger office.
3) Create a VPN tunnel using these firewall/VPN endpoints.
4) Assign DHCP for the smaller satellite office from the local firewall (PIX/ASA)
5) Install a windows terminal server or terminal services install to the main office server
I would avoid worrying about failover for cost/complexity reasons as it should happen very infrequently if at all. The terminal server would be the primary or backup for the office in case the DSL/VPN tunnel down. If the office still had internet access on the other internet connection, you could just log into the firewall, change the DHCP options to give an internet DNS server and then have the clients reboot which should give them the new settings, allow them internet access and then just log into the MS Terminal Server and run their apps (and map drives) from there until the VPN tunnel can be brought back up.