[CentOS] per port ip routing -- possible?

Hello,

I have a customer who has two Internet connections, cable and DSL, where 
the cable is much faster than the DSL connection.  Is it possible in 
CentOS 4.2 to use the "ip route" command to direct certain outbound 
traffic over the faster connection, say http(s) requests?  Web searches 
turn up solutions where NAT is involved, but this guy isn't using NAT.  
Example configs that show how to load balance multiple connections and 
ensure symmetric routing all work great, but I don't an example of 
per-port routing.

Any pointers or example commands would be greatly appreciated.

Jack