http://xbtt.sourceforge.net/ - XBTT
http://www.utorrent.com/ - uTorrent
Both of these have very low overhead and I have found them both to be very responsive.
You can keep your running torrents, if I'm not mistaken if you store the files in the same place and use the same torrent files in the new programs, you shouldn't have any problems continuing your downloads. YMMV.