You can access your PC behind the NAT'ed Linux box from the internet. Try using these commands:
iptables -t nat -A PREROUTING -p tcp --dport 80 -i eth0 -j DNAT --to 5.6.7.8:8080
This should send all the requests you get on the server that does the NAT on port 80 and sends them to the machine with the IP 5.6.7.8 port 8080.
If you're interested in NAT
this is a great HOWTO.
