What I was trying to do, and did, was to be able to use my Linux box, which has it's own static ip address, to forward it's ports to Windows XP machines so that our sales guy's can log into their computers with "Remote Desktop Connection." I especially wanted to be able to do many Windows machines with just one IP address by using different ports. I ended up using ports 3390 and 3391, each port forwarding to a different Windows XP box.
This is what worked:

 #Flush tables
 iptables -t nat -F
 iptables -F
 
 iptables -t nat -A PREROUTING -i $EXT_IF -p tcp /
--dport 3390 -j DNAT --to-destination 192.168.0.88:3389
 
iptables -t nat -A PREROUTING -i $EXT_IF -p tcp /
--dport 3391 -j DNAT --to-destination 192.168.0.106:3389

 iptables -t nat -A POSTROUTING -o $EXT_IF -j MASQUERADE