It looks like the machine I have (an HP Kayak XAS, you may have seen me
mention it already) can be upgraded from a PII 400MHz to dual PIII
600MHz.
But the PIII's are substantially more expensive than the PII's, around
$70
each compared to $10 each.

I guess I'm not that excited about a 50% increase in clock rate, by
itself. But is there a great advantage just in going from a PII to a
PIII
with a comparable clock speed? Would I get substantially more computing
for the money?

There *can* be an advantage, but whether it's important to you is
debatable. Early P3's had the same cache architecture as the P2's -
namely, external cache running at half of the CPU's clock speed. Starting
around the 600 MHz mark, you were able to get P3's with on-die cache
running at full CPU speed.

So, if cache latency is a deal-breaker for your application, the extra
money would be worth it. If not, then it's a lot more of a
personal-preference type of thing.

As a side note, I'm surprised that a P3/600 would be $70, seeing that
you can buy something like an Athlon 2400+ for less than that. I looked
over on ebay, and found the P3's running $20 to $30 each.

steve