I tried using several versions of the BitTorrent client on an iMac Dual Core 2.0Ghz, and all versions gave me trouble in some form or other. I know that a lot of people enjoy Azureus, and it has been recommended several times here, but from my first hand experience, Transmission is by far a better and more stable application. If you've had some issues with other torrent clients, give Transmission a try!