forward ports 6881-6999 in ascending order to your comp. Also keep an eye out on Shadow's core, hes working on multiple connections over a single port for the future, a nifty feature for people with limited port forwarding abilities.

I have yet to see a client that allows you to specify which ports to use for a given torrent, unless its ABC. I haven't used it for awile now so I'm rusty on its feature set.

Of course you can always download as many as you want, but your speed will be greatly decreased by your inability to connect to people who don't have all the torrents data. even if they have chunks of the file you don't.

EDIT: Clarity