All of the clients I have tried so far (Gnotella, Limewire, Bearshare, Fury, to mention only the ones I actually used for more than half an hour!) locked all of my system memory at a certain point (didn't USE it, they only LOCKED it!)

While Gnotella was still the most usable client (I could even manage to d/l some 600Meg+ files with it), it still wasn't possible to actually run it in the background.
The only chance to finish big d/l's was to run the client and don't touch the system anymore.

Is this something that cannot be avoided? No other application ever used that much system resources!

I might mention I have an Athlon 1000@1040, 384MB RAM and I am a power-user, I finish a Seti@home workunit in 7 hours and get 5875 points in 3DMark 2000 - and not to forget: I'm on a 10Mbit LAN connection...