If you don't forward the bitcoin port from your router or disable upnp in your bitcoind you'll only be able to make outgoing connections to the network, that will stop bitcoind from using any real upload bandwidth (except when someone in the p2pool network finds a block which is only about 3x/day).
Or just set your max connections in your bitcoin.conf. I set mine to 10 so I can still have the port open without my network getting bogged down.