It all depends on the number of active users on the torrent (and of course how much they're uploading), and let's not forget the matter of setting up your router properly. This will mean the difference between a d/l speed of 5 KB/s vs. 100-200 KB/s all depending on the circumstances... Personally I've just opened DMZ for my computer cuz I'm not that much into adjusting individual ports, but that means any sort of firewall protection within the router is automatically bypassed...
Edit: Forgot something. Be sure to adjust the maximum upload speed to a proper setting as it will otherwise hinder the incoming data stream. This is due to that fact that when your computer recieves data it needs to confirm with the sender. If you're consuming your entire upload bandwidth this will take extra time, thus lowing the overall D/L speed. For 128 kbit/s I'd say around 11-12 KB/s would be a good maximum for uploads. If your BT client doesn't support bandwidth limiting, try a nice app called NetLimiter.
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