Firewall and VLAN security design

[Ahmed Ameen <ahmedameen () gmail com>]

Hi All,

Currently we are redesigning our LAN to include a DMZ zone, and we
need to reach the best security design.
The available equipments are:
1-PIX with 3 NIC's
2-L3 Switch
3-N-IDS

My preliminary design is as follows


Internet
    |   
    |
--------
|PIX   |____DMZ
|        |
--------
    |
    |
   LAN




Internet
    |   
    |
--------
|NIDS |____DMZ
|        |
--------
    |
    |
   LAN




Internet VLAN1
    |
    |   
---------------
|L3 Switch|____DMZ VLAN2
|              |
----------
    |
    |
   LAN VLAN3

My Questions would be:
Is it ok to use a multi homed firewall, or should I conceder 2
physical firewalls, what would be the threat of using one.

Is VLAN segmentation enough to segment between the internet, DMZ and
the internal network, or should I also use different switches for
each, and be connected through the firewall.

Thanks

Firewall and VLAN security design