Gnucleus is probably the best Gnutella client, the latest version is pretty much the same as Kazaa/Morpheus was in it's heyday, you'll get good results.
That said, I rarely bother with keeping downloaded mp3s - most people do an utterly shit job of ripping, and there's all those 128kbps mp3s floating around. Best way to get MP3s is to buy the disk and do it yourself. ;> But gnucleus and net radio are great for finding new shit ya never heard of.
Cheers,
Backov