Gigabit card is only useful if you also have a Gigabit switch and another Gigabit machine on the network, and you are streaming files between those machines. It will not help with BitTorrent because your internet connection speed is almost certainly far less than the 100 Mbps limit of Fast Ethernet.

Even HD content is usually not more than 20 Mbit/sec so you could have four HD streams running smoothly over Fast Ethernet.

With that said - if you have a Fry's nearby look for the AirLink Gigabit PCI Card. It's usually dirt cheap and the chipset is compatible with OS X out of the box (personally tested on Leopard - probably works in Tiger as well).

Transmission BitTorrent client has a web interface.