You may set up both cards by editing /etc/rc.d/rc.inet1. Also, make sure that IPV4_FORWARD is set to 1 in rc.inet2 (it is by default, so if you haven't been poking around already you probably don't have to edit it
). The tutorials linked to from netfilter.org should make masquerading a breeze. Good luck.
(I might as well mention that I ran a similar setup more than a year ago, but I don't anymore...so if my information is not up to date or something, that's why.
)