The problem with OSX & LW has always been Java. OSX's implementation of Java has not been very good. People had been crying out updates to Java. Actually Mac have released a later version but is only available for Tiger users. I've been told it does improve performance somewhat. I don't know how much because I still use Panther OS 10.3. But may I suggest you update your Java to 1.5 & then use the java preferences utility (found inside the Utilities folder) to force LW to use Java 1.5 instead of 1.4. LW will not use Java 1.5 unless you tell the Java to make itself available for LW.

Follow all the directions here: http://docs.info.apple.com/article.html?artnum=302412

I must say, LW's cpu usage on your computer is considerably low. Mine tends to be much higher.

About your LaCie external drive, do you find it slows your computer down at all when you connect it? Do you use it to share your music with LW? I ask because I use the same size LaCie drive & I've had nothing but problems with it. I wrote a very cheeky thread about it here: LaCie 160 Drive Problems (click link) Where it is not only unreliable but slows my computer down when I share it. It believe it also resulted in some of my programs freezing. I only use it occasionally for sharing now. I'm still trying to decide what to do with it. Try an experiment. Try not connecting it to your computer & using LW. Do you suffer the same problems with it not connected?

Here's some other ways to reduce Java's high usage of Virtual Memory (VM) & RAM:

Keep the all of LW's windows closed unless you really need to use them. eg: The connections window & the Library window. Don't use the Show incoming searches in the monitor window unless you're running as an ultrapeer.

Set all uploads & downloads for auto-clear; ie: the drop off the download/upld list as soon as they've completed downlding/uplding.
Close off some search bars if they start to get into 2nd row & if they're relatively old searches. If you're not in front of your comp, then hide LW in the dock. Also Don't use the Resume button on all incomplete files. This can have side-fx such as affect LW's connection quality. May queue your files for much much longer. This will also stress LW & your comp. If you use Resume/Find sources then just apply it to a couple or so at one time. After opening LW wait 10 to 20 mins or longer before using. Let it settle down 1st. Lastly, one option you might consider, which I very rarely do is Tip: Purge the Downloads Queue!