Looking to purchase...

...a hardware firewall for an office network. We have about 15 computers connected through a DSL modem plugged into a switch. I was wondering what exactly "unlimited nodes" stands for on firewalls such as our old sonic wall? Will this firewall protect the amount of computers we have? http://www.zipzoomfly.com/jsp/ProductDetail.jsp?ProductCode=251800
http://www.netgear.com/Products/VPNandSSL/WiredVPNFirewallRouters/FVS124G.aspx?detail=Specifications

"Unlimited nodes" means exactly that - the firewall will work with any number of hosts behind it. This is opposed to products that will work with only X amount of computers at a time.

As for which device to get.. This depends on what you want that firewall to do and how much traffic it should handle.

PS
I really doubt that you would have functional internet with lots of computers plugged into a DSL modem. Are you sure that this "DSL modem" thing does not already include a router and therefore also a firewall?