connecting to internet and intranet via switch

Hope I can get help on this topic.
Our company have several depts. where they r connected on Lan for intranet and internet . One of the dept we are having is customer service ,

But now customer services dept want to use a dedicated internet line to their pc?s but still they will have to have access to intranet for internal applications.

So I planned to do this in following way?.
Got 3com 4400switch , I connected all pcs to that lan and internet works fine.
Got a dedicated BB line , but I don?t know how should I configure this switch in way that it uses this Broadband line for internet and Lan for intranet .

Any help will be appreciated..