dump Transmission, and use Bit on Wheels, it much better.
Naa, I courtiously disagree.
B.O.W. isn't being actively developed any more, and isn't a universal binary.
Sure it still works, but it isn't exactly kept up to date, or resplendant with features.
Transmission post v 7 (I've been using the daily builds for a while) is a great app.
And if your router supports UPNP and it's enabled, transmission can sort out all your port forwarding without you lifting a finger (cue some "security expert" saying it's a bad idea to keep upnp enabled, but I say go for it, just make sure your router Firmware is up to date).
PEX is peer exchange. lets you download off people faster apparently.
Your client asks clients it's downloading from what (good) clients IT'S downloading from, and adds them to you. Meaning all the guys who are playing nice interconnect with each other.
Hope that helps!