I'm shopping for a firewall appliance for a small/medium organization (about 30 users and growing). We were looking at a Watchguard Firebox 700, which looks great, but I would really like something that I could add antivirus filtering to. I'm looking into the new Symantec offering, but that's a little too new for me to be comfortable offering.

What have you all used to this effect? I'd like to implement a multi-layered approach to my AV defense as soon as possible. Thanks!