internet + LAN through same switch = security issue?

Hi,

what is the security implication if I put the traffic from 2 subnetworks on
the same physical switch?

I use a dedicated ISA Server computer with 2 NICs for my firewall and VPN
access. (1 nic connected to the net, the second to the LAN)
This ISA Server is my default router for my internal users.
So for my actual location, the internet and LAN networks not share the
physical cables. (1 switch for the net and another for the LAN; only my ISA
Server share the 2 subnets.)

Our company will move, and for the moment I've a setup issue for my network.
all my local network cables go to a communication room (where I have the
phone system and the internet router)

Because I keep my servers in another room and because from this second room
to my communication room I have only 1 network cable, my internet trafic and
LAN trafic will go through the same cable and the same switch.

My actual switch is unmanaged (a linksys one)

So I presume there is a security issue (?)
My internal users will continue to use my ISA Server as the default router.

for the moment I have 2 options:
option 1: new cable from the comm. room to the server room
option 2: puting the ISA Server in the comm. room directly

I think the option 1 is the better way.
option 2 cause some problem (due to the comm room access and future
extensions)

But before this, I want to know the security issues of sharing the same
switch.
and I want to know if I can use a different setup?
Does a managed switch or router allow me to control where the traffic will
go.

thanks for your comments.

Jerome.