PS - we actually have an ISDN connection on the ISA server too (as a third line of defence) so I guess that's the same as 4 NICs / 3 internet connections.
ISA server interacts with RRAS which does the routing. I'm having problems with connections (particualrly pc anywhere) dropping off though, and finding it very hard to tell which part of the set up it is. This set up seems to be totally undocumented, so if anyone else is using it I'd be very interested to hear from you!