If these assumptions are correct, then I don't see any need for separate VLANs. If all your users are in the same IP subnet, then having multiple VLANs can actually degrade your performance, since inter-VLAN routing will have to take place every time users try to access servers.
My suggestion would be to analyze the traffic on your network, and ensure that the broadcast is not very heavy. Also ensure that none of your network segments are currently congested. You can do this with any free packet capturing utility like Ethereal. If you have a Sniffer license, then that would also be a good option.
Dig Deeper on Network management and monitoring
Related Q&A from Nandan Gijare
Is it possible to connect PRI interface using a E1 link? Continue Reading
Can I connect two computers through Ethernet on a WAN? Continue Reading
Create a network of three computers running a Windows2000 server. Continue Reading