Xbian still my favourite RPi OS

[zilexa]

I have limited the number of downloads to 2 and uploads to 1. And set down/up limits of 400/50 KB/s. This way I can still use XBMC while Transmission is downloading, but its slow, not a problem cos playback is fine (only tested with SD content so far). My USB harddrive is NTFS. But since I am going to download to EXT4 soon I suppose down/up limits can be raised and it should still work.

So I would recommend to limit Transmission a bit so it won't choke the I/O interface and you'll be able to SSH.
Also a good idea is to limit the seeding ratio. Since Transmission has no way to set when you actually want to stop torrents completely (uTorrent is much more evolved in this featureset), your only way to automatically stop torrents from eating your upload limit is to set a low ratio.

Otherwise you will have a lot of seeding torrents which will choke it up again (unless you clean the queue manually regularly).