When testing for port forwarding, a good idea is to eliminate all unknown, so in your case I would do the test with the firewall disabled.
Regarding the port forwarding, are you using static IPs or DHCP? Sometimes it happens that routers allow you to port forward on the IP, but when the machine is setup to use DHCP, on the next reboot it may have a different IP and everything fails. (In this case it’s better to set up a fixed IP on the router by associating it to the MAC/Hardware address of the network card on the PC)
I assume the machine has only one network card?
Just to be sure: When you test the port, you don’t have the server or game running?
Regarding the 6 gigs of ram, I fear you are not going to do much with the machine, it’s very limited…
I’m not sure I understood this question 