I've used pfSense in a number of deployments and have been thrilled
> with it.  Does Multi-WAN very well.  I haven't done multi-wan with
> only one external interface; you probably need a NIC per IP.  I (and
> probably most on this list) have fistfuls of spare 10/100 cards that
> could be donated to your cause.
>
>
>   

I am looking over pfsense now. In your experience is there an issue with 
requests coming in on one interface, returning via the default route 
(different interface)? I see some mention of it on their docs, and not 
having set up a multi-wan firewall before, I am not sure if it will be 
an issue,.All traffic will return to the same ISP gateway (I think).

Waiting for the installer now.