help with routing

I have a RedHat 9 box with the following, Postfix 2.1.5 and iptables and
NAT, Squid, snort, privoxy , bandwidth limiting. BIND

the system is use to access Internet and block some sites, now the SMTP
server use to relay mail form clients only. (out bound)

now we are plaing to add a new ADSL link for web browsing only with
dynamic IP, and the other link has static ip with reveres DNS form the
SMTP server. now i need to do the following. to have the new link only
for the web browsing and the old link for mail server only. (SMTP relay) 

how can i do this with one box ?

thanks
kev
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