Are you planning on all the computers being able to get on the internet? If so, you're going to need a router before the switch. Unless you are going to use internet sharing, which would be interesting with that many computers.
Edit: Did you read the last review on the page for the firewall?
Reviewed By: Farslayer on 11/2/2006 Tech Level: high - Ownership: more than 1 year
Pros: Despite other reviews, this device is a firewall with routing capabilities; it is not a router. Do not buy this product if you are not a network technician as, yes, it requires you to know what you are doing before you can use it. Configuration is easy through a web interface. Supports WAN, VPN, Gateway Intrusion Prevention and AV, network AV (with upgrade), DMZ, customizable rules, content filtering, etc.
Cons: This device can be deceptively pricey if you are not familiar with them. The basic device works great, but you may need to invest in upgrades which have recurring fees such as IDS. I recommend upgrading to the latest OS via their site. THIS DEVICE IS NOT FOR NEWBIES!