LOL it shouldn't be slugish with 300ghz ;)
On a serous note: post the output of top.

I think it would be very sluggish. The first thing you try to do would take years waiting for your model of CPU to be invented.

That would the explain the "Please wait while the product development cycle completes" messages I've been getting!

QUESTION, then: if I'm springing for the memory, what configuration would be best?
Two 1 GB sticks? 3? or all 4?
1 2GB stick? or two?
Thoughts?
Thanks in advance!

I will post the output of "top" shortly for your perusal...

To get decent ram performance, the ram should be in matched pairs of sticks, so don't buy one or three.

If you bought 2x2 or 4x1 and replaced all the old ram, I don't know how much you could actually use. My guess would be about 3.25 of the 4GB would be usable.

I've seen very contradictory information about the memory limits forced by the Intel 945G chip. The motherboard/BIOS couldn't give you more ram support than the 945G permits. It might give you less.

2GB or 2.5GB ram might give you a significant performance boost from 1GB. The upgrade to 3.25 (or even approaching 4GB if I'm being too pessimistic about your motherboard) would cost almost twice as much as the upgrade to 2.5GB and I don't think even 4GB would be twice the performance improvement of 2GB from 1GB.

2x2GB lowest cost from NewEgg is $88 with free shipping:
WINTEC AMPX 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model 3AXT6400C5-4096K

I don't know how careful you are with money. Maybe $88 is little enough you don't care if you might only be able to use 3.25 or 3.5 GB out of 4 and you won't get near double the performance boost that 2x1GB for $49 would do.

Right.

Still, 3.25 is, to state the blindingly obvious, more than 50% better than 2...

Presumably you have decided to replace the 256 kB chips so you can move memory speed up from 533 to 667.

Question to consider: will the choice influence the useful life of the memory?
Factors to consider:

• Larger sticks will be useful longer.
• DDR2 800 is coming to the end of its market dominance; hopefully your motherboard will last until DDR3 becomes commonplace.

Question to consider: what is the optimal amount?
Factors to consider:

• A 32-bit system can only address ~3.5 GB although this does vary from distro to distro and depends on RAM used for video (as zeno0771 pointed out your motherboard does if not fitted with a graphics card).
• Enabling PAE to overcome the ~3.5 GB limit only makes sense if you have say > 6GB memory and use it because PAE itself incurs a ~5% memory access penalty.
• An example discussion here.

Question to consider: what is the cost of the various options?
Factor to consider: typically larger memory chips are cheaper per bit than smaller ones.

Factor to consider: having more memory chips reduces the impact of single chip failure (once it's removed!) and makes it easier to investigate memory problems by swapping chips around.

I don't think so. Because
I disagree. The performance cost of PAE is very hard to measure or predict, but generally is a small fraction of 1%. Estimating 5% is absurd. But I think that motherboard might force the same limit as 32 bit non PAE, so even a 64 bit CPU and kernel would still have the same limit in that motherboard.

I'm not sure about the motherboard limit. If you get 4GB and the motherboard allows you to use all of it in a PAE kernel but only 3.5GB in a non PAE kernel, I would definitely choose PAE.

The question is: will I be able to get a significant performance boost by adding a given memory configuration?

If so, I can live with this computer for some time to come.

Also, dmesg indicates that I have Linux 2.6.26-2-686.
Is that PAE capable? If not, is installing a PAE capable kernel problematic?

Thanks for the correction :) Perhaps I misremembered or misunderstood ~0.5% as ~5%.

Were you going to comment on DDR2-800 coming toward the end of its market dominance?

We're just guessing at that important question and there is no new info here with which to improve the guesses already made earlier in this thread.

I don't think that is enough info to tell whether it is PAE.

In Red Hat based systems the config file with all the kernel build choices is
/boot/config-`uname -r`
So the command
grep 64G /boot/config-`uname -r`
Would tell whether a 32 bit system has PAE. I'm not sure how similar that would be in Debian.

If PAE would help, installing it should be no problem. As I explained before, if you get 2.5GB total ram PAE would have no advantage. If you get 4GB total ram PAE might or might not have an advantage depending on details of your BIOS/Motherboard that I don't know.

I guess I was unclear. I put that quote after the word "because" in my reply intending to show that quote finishes my sentence, meaning you asked the "useful life" question but then included what I thought was a sufficient answer to that question.

Too many variables, too little hard data.

How important is the switch from 533 to 667? If it isn't important buying 2x1GB gets you 2.5GB. If it is important, buying 2x1GB just gets you 2GB. How important it is depends primarily on the L2 cache miss rate of the programs you run. We have no clue what that miss rate is.

How many disk I/O's per second would be saved by having an extra 1GB, vs. an extra 1.5GB vs. an extra 2.25GB etc. We don't know, but generally it should be a declining function: 1.5GB extra should save less than 1.5 times as many disk I/Os as 1. 2.25 should save less than 1.5 times as many as 1.5.

None of that is rock solid predictable. With a little effort, you could construct a scenario in which an extra 2.25GB of ram saves far more than 1.5 times as many disk I/Os as an extra 1.5GB. It just isn't very likely.

Are disk I/Os due to lack of ram even a significant factor in the current performance? We don't even know that.

Isn’t this getting overcomplicated a bit?
DDR2 800 seems to be the same, if not cheaper, than DDR2 667.
So he should pick up 2x1 GB DDR2 800 (such as WINTEC AMPO 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) - newegg.com/Product/Product.aspx?Item=N82E16820161172), remove all the older RAM and install the 2GB. Then he can check the performance, add back 0.5GB (2x256MB) and check the performance again. If slower, remove the 0.5GB of old RAM.

Then if he wants to buy 2 more GB of RAM he can always look into the PAE thing.

Right?

I would be a little skeptical as to whether a P4 board will support 2GB sticks. It's not unheard of, but I would look closely at as much documentation as I could find to make sure the board will in fact support those sticks. On a board of that age (I know it's still useful, please don't flame me) I would be real hesitant at making memory the main upgrade. I saw a core2duo quad core e7500 and a gigabyte motherboard for $87 dollars at fry's this week. If I was considering a big upgrade it would be at the cpu/mobo level. 4 GB pc6400 RAM at the same place is the same price.

Just my two cents.

I agree the board may not handle 2 gb sticks.
1 gb sticks should shurly work but its not my descision so...

Correct, RAM only runs as fast as the slowest stick in the board (or the FSB whichever is lower); thanks for pointing that out.