I personally prefer to use the 10.x.x.x network for my networks. You could also use 172.16.x.x - 172.31.255.255 and of course 192.168.x.x.
I recommend that you create VLANs (preferably trunked as this requires less switch work), however you can setup the ports to be static but if you move devices you will need to reconfigure the switch. All server NICs have the ability to specify a VLAN ID so ensuring the server is on the right VLAN is simple.
As for VLAN design, I would recommend that VLANs be created based on location, and then function. The table below is a quick example of how you could divide your network into VLANs.
Function Location VLAN
Servers Extranet 10.50.x.x
Servers Internal 10.40.x.x
Clients Wired 10.30.x.x
Clients Wireless 10.20.x.x
Guests Wireless 10.100.x.x
Larger organizations will also separate departments into their own VLANs, for instance, Accounting, HR, Development, Engineering, Management, etc. With 400 users this may be more work than it is worth.
Another benefit of VLANs is simplified firewall management. If you have servers in the Extranet that you don't want to reach the intranet, if they are on different VLANs you can simply block traffic from the entire subnet instead of one by one.