QUOTE (TeknomanFan @ Jun 29 2008, 08:36 AM)

Or does BT only look at you total upload speed and decides on your download speed just by that?
It doesn't do anything like that. The thing is that most clients have "anti-leech" protection which couples download speed to the upload speed you're making available. Ergo, when you lower your upload speed it'll limit your download speed as to protect the swarm from leeches who download but don't upload.
There are clients, like BitTyrant iirc, that have very low upload settings while getting high speeds from the torrent swarm. It's why those clients are usually banned from the trackers as to protect the other people using the torrents.
But, there is something which can have a huge impact on your speeds: Throttling. There are ISPs out there that throttle download speeds from torrents as to discourage people using it due to the massive amount of connections opened by torrents. To beat this you might try to use encryption which is currently provided by most, if not all, of the major torrent clients.
And on the note of speed: Fastest speeds are attained by getting access to (private) trackers which enforce share ratios to ensure a torrent stays healthy.