Instead of multiple PCI slots, you could try a PCI card with multiple 
ethernet adapters on it:

http://www.routerboard.com/rb44.html

RouterBOARD44 PCI 4-port Ethernet adapter.

James W Harrington wrote:
> I currently have a m0n0wall running on an old Dell Dimension XPS (P3 1Ghz, 512MB RAM, 40GB Hard
Disk).  I knows it a bit overkill but it's all I had.    I am now looking at upgrading as the old
hardware is starting to become unreliable.  The problem is I need 6 network interfaces :
>
> 1.       Onboard used for WAN Connection
>
> 2.       PCI Slot One to Four using 3com NIC's
>
> 3.       PCI Slot Five has a 3com Wireless PCI Card
>
>  
>
> Dell don't provided a PC with more than 2 PCI slots that I can see despite asking them.  Does
anyone know where I can get some kind of not too expensive hardware (around £400-£500) that would
fit this requirement.  Failing that I would consider using VLANS but I have no idea how to set them
up.
>
>  
>
> Thanks for any help
>
>  
>
> James
>
>
>