If you want to use bittorrent, Azureus is the client of choice for faster machines, and μTorrent if you want something that barely takes up more system memory and resources than Calculator or Notepad
If you have one of those routers that have an extremely limited memory chip for NAT (network address translation) Tables (like most SMC, Linksys and D-Link modemrouters), and freeze up when they get saturated by bittorrent connections quickly, get BitSpirit - it's not the most stable, but it has a DSL anti-freeze option.