Add another network segment in ofice lan

Hi,
We have 10/100mbps ethernet lan running on Cat5e cables using 3Com switches.
Presdently we have 192.168.100.x segment with our Firewall on lan port 192.168.100.254 as our default gateway for all systems.
Now I plan to add one more lan segment 192.168.101.x along with it as well.
I want that the users on 192.168.100.x & 101.x should be able to see each other.
I think I need to use a Cisco Router to implement this setup & connect the two 10/100Mbps networks plus have 192.168.100.254 asour default gateway -for both the network segments.
Advice ???
Akash