I did a google and it appears that the MN-500 is a wireless 802.11b router. Now you will need a wireless 802.11b PCI card to connect to this with. Just put the PCI card in your second computer and if you're running windows xp, run the wizard to automatically connect.
If you don't want to go wireless you have a couple of options. Just sticking with the wireless router you already have might be cheaper and more sensible since you only have to buy a wireless PCI card to connect to it. However if you're planning on doing gaming or some other bandwidth heavy task on the second computer, maybe wireless isn't a good choice.
I couldn't find very much info on your wireless router, but if it has four ethernet ports on the back you could just run wires to both computers. If you don't have enough ports to go around, you can connect a switch to the router. Don't use a hub, a switch is better.
In the end it depends on what you want to do. You already have half the wireless setup, so it'd be easier in this case. Plus a no wire setup is always nice. However you have to take in mind that an old wireless router like that probably won't have the most up-to-date security options. Also it is 802.11b, which means it's going to be a little slow, especially for gaming. You'll be fine if all you are doing is web browsing/ email. Good luck with whatever you choose.
