Well, it run's on my system (SuSE 7.3 Athlon TB 800Mhz, 256mb ram), using Sun's jre 1.4. No matter if I'm running as an ultrapeer or not and sharing ~6000 files.

There might be problems while LimeWire is hashing your files. I don't know anything else what could be causing your problem (except for maybe more than a hundred downloads at the same time).