Small office network and fileserver questions

I work for a small company that is in the publishing biz. (ie: lots of BIG graphic files and ALL Mac) We're moving from a shared office space that has its own network, to our own digs and will be running our own network. I've had varied experience with networks, but never something so encompasing as this.

Here's what we currently have:
4 PowerMac's
3 Laptops
Possibly adding 4 More PowerMac's

What we need:
1TB Fileserver/NAS RAID (not sure which type 0-5)
10/100/1000 Router to handle incoming Business DSL line
Not sure if we need a firewall

I've looked at various NAS devices, but am not sure which one would be the best to suit us. I'd like to have a 10/100/1000 network connection to help the transfer speeds, as a lot of the files we deal with are 4+ megs each. The other concern is keeping the information on each of our computers and especially the Fileserver, safe from outside users.

If anyone here has dealt with this kind of network before, I'd sure appreciate the help.

Thanks
-S