A Layer 3 switch means that it has IP Routing functionality built in for Ethernet layer traffic.
With only 40 computers, I don’t see any need to subnet your network. Hence, I don’t see a reason to use the Layer 3 functionality.
If it were me, I would go with a 1GB switch and get something that you might really use. If your PC’s don’t have 1GB NIC cards, they are less than $30 each these days. I would put one in your highest traffic server and get the Gig-E switch.
David Davis, CCIE
ITKE Blog – Cisco Router & Switches