Hmmm...more good suggestions to think about. In particular about looking into XP hacks and tweaks. Even though I'd have to learn the ins and outs of a new OS (I have the workings and tricks of 98SE down solid...) that might be reasonable.
JD: you are right, there _should_ be no reason to run 98SE in 2006

There are lots of examples of XP user-control issues or limitations that could be listed, but that's OT so I won't get into them; here's one example though: when using a projector during presentations, the inability to display a video on both the computer and projector--XP will only allow one copy to run at the same time. Or, taking screenshots; media will not be captured. I understand the reasoning for all this, and I've heard there are workaround, though, but a user shouldn't need to take the time to 'fix' such basic things:)
The reason to switch from the 600E is due to processing speed. As mentioned, some programs (such as Matlab) just take too long to execute their code.
Anyways, another intriguing suggestion was that of using 98SE in a VM...clever. Would that regain all the functionality, but with the rock solid stability of XP?
I also like the idea of the 600X, which I hear can be 'upgraded' to faster processing speeds as well. But, as mentioned before, that would probably mean the cooling fan is on quite a bit, which is not preferable... a silent machine allows much better conentration at work.
So thanks for the ideas so far. The choices now are T23, T41, and 600X now; the T41 is slightly ahead.