Routing through multiple interfaces based PORT?

Hi, wonder if anyone can tell me if this is possible and if i can do
it iptables or i would have to do something else,

what i want to do is basicly seperated traffic over 2 internet
connections based on which port it is going on.

I have 2 interfaces on my Linux machine, one external with a public IP
address, one internal.

Traffic is coming in on the internal interface and being masqueraded
onto the net, pretty standard setup, but there is also a DSL router on
the local network, I want to be able to force some traffic to be
masqueraded back onto the internal network and through the DSL
router(based on port, ie I want all http traffic to go through the DSL
router)

For example

When my client(192.168.1.10) wants to establish an ftp session. Its
default gateway is set to (192.168.1.2) which is the Linux machine, it
then masquerades it and sends it out on its external interface
(65.xx.xx.xx). Thats fine, but if my client trys to establish a
connection with a HTTP server over port 80, Can I make the linux
machine masquerade it back onto to local network and out through the
DSL router(192.168.1.1). I can add 2 default routes in Linux but I
think that then makes Linux auto select which one to send it to?? Im
not sure about this.
??

I hope is clear what I want to do....