|I have the same setup as you. Here's what I did.|
I set my linksys router IP to be 192.168.2.1. It then set the network to be 192.168.2.x. On my PC, I have it a static IP of 192.168.2.114. This is done in the properties of Network Connections, then Properties of your Local Area Connection, then properties of TCP/IP.
Then I go to the advanced routing and look for a forwarding tab. I Forward port 3389 to the static IP of my PC.
OK, but what if your ISP changes your IP address? How do you prevent that. Well, sometimes you can't control what the ISP does. However, most DHCP servers will keep the MAC address of a client computer in its memory for a set amount of time. I then went to the Clone MAC tab and entered in the MAC address of my static computer's Network Card. That, in effect, gives me the same IP, even if my cablemodem resets.
If your ISP is hard headed, and doesn't like that, you can install a Dynamic DNS proxy program, such as DYNDNS on your PC, and instead of connecting to your IP address, you could connect to the DNS name you setup.