What is the significance of supply voltage in RAM?

The voltage is a design parameter => not directly linked to speed, although higher voltage is one way a part can be designed for faster switching speeds.

As for why DDR memory costs more than DDR2 ==> simple supply & demand economics.  Think that's bad ... check out the even-more-obsolete SDRAM modules :-)

If you buy the right RAM for your system (i.e. right DDR vs DDR2) it will be the right voltage, which is not related to speed. What IS related to speed is the CAS LATENCY -- that is how fast the ram needs to be refreshed to stay alive, the smaller the CL number, the faster the refresh is.

The fastest MB chipsets, like Nvidia, often will not work with CL 3 -- the refresh is just too slow for the motherboard chipset, i.e. the RAM cannot keep up, so you have intermittent freezes due to Ram dis-synchronization refresh.

Always get the FASTER CL ram refresh -- get CL 2.5 -- it is significantly faster than CL3, and it will work with all the high performance motherboards -- you did not state your MB chipset.  Anyway, get CL 2.5

My motherboard is the Asus A7N8X-E motherboard.  

FOr any A7N8X ASUS motherboard, you will need CL 2.5 -- that is the ONLY crucial factor, despite the long rambling diatribe above.  Buy CL 2.5 RAM, and keep it PC 2700 if you can (PC2100 is OK, but the board is matched for PC 2700 RAM and CL 2.5) -- and it will work.  ALmost ALL CL 3.0 RAM will not work on this board, I am an expert on them, I have installed over 200 and these are the facts of that MB.  Good luck.

In general, a higher voltage usually means that it is possible for the circuit to perform faster (because it reduces the effect of capacitance - the "resistance" to change in voltage). On the other hand, higher voltage usually means more heat. Another thing to consider is a  higher voltage is harder on the motherboard power regulators. If all banks are populated with high-demand memory, the motherboard power regulation might not be able to respond as quickly to sudden changes in current draw, possibly reducing stability.

In my opinion, memory is the most important thing for stability. Memory problems can be very hard to pin down - you might have bad memory that seems to be ok for a while, but suddenly fails when you do something different with the system. Don't try to get high performance with really fast memory, I/O performance is far more important than memory speed. The CPU contains caches that hide the slowness of memory almost entirely. Doubling memory performance will not even come close to doubling performance when running normal software. If you want speed, look at increasing performance using RAID-0 or RAID-5 disk interfaces.

I'm currently using PC3200 memory rather than PC2700.  CL 3.0 speed ram has worked in my PC, though I am using CL 2.5 at the moment.

Remember also that CAS latency is a measure of the delay between a column address signal being placed on the bus and the data being ready => so how many clocks are needed is also a function of the speed at which the RAM is actually being run at.   So if you're using PC3200, CL3 RAM, the actually latency setting will be lower if the RAM is being clocked at PC2700 speeds.

If you want to know the actual parameters your RAM is running at, download CPU-Z;  extract the files to a directory; and run it [It doesn't "install" anything or write anything to the registry]
http://www.cpuid.com/cpuz.php

The Memory tab will show you the actual parameters your memory is currently running at.   The SPD tab will show you what those parameters are for other speeds.

ok update.

After returning some v.old faulty 2x 512mb twinmos modules, I now have 1gb module 2.5 CAS speed DDR 400, 1gb module CAS 3.0 speed (different make but still DDR 400) and 512mb Geil DDR 400 CAS 2.5 (soon to be replaced with another stick of the exact same first 1gb module.

All modules have been tested by me and are fine.  Will mixing the different a) sizes matter and b) different CAS speeds matter even though the system works reasonably fast and does not crash?

Also the 512 mb is not being recognised when combined with the 2x 1gb modules.  Why is that and will I have the same prob when the 512mb is replaced with a third 1gb module?

Thx.

Mixing the modules should be no problem => they'll just operate at the lowest speed and highest CAS latency that's common.

Are you installing the modules in the correct order?   The two 1GB modules should be in slots 1 and 2; and 512mb should be in slot 3.   Your slots are numbered   <1>  <3>  <2>  <4>

Some older memory controllers REQUIRE matched pairs => that shouldn't be the case here.   You can confirm that by trying a single module in Slot 1.  

I only have 3 slots on my motherboard.  If I want to use dual mode, I need to put in 1 and 2, but in my case all three slots are in use with 1gb in each slot 1 and 2, with 512mb in slot 3.

Okay ... I noticed you mentioned the motherboard module during the thread ==> I had assumed you had an Optiplex GX270, since your initial link in the question shows that the memory modules you referred to were for that model.

Does the 512mb module work if it's installed as the ONLY module in the system?

The 512mb module works when installed either alone or with ONE of the other 1gb modules (thereby totalling 1.5gb), but doesnt seem to work when included with two of the 1gb modules together.  

Are the modules different densities?  (single-sided vs. double-sided)

double sided (chips on both sides...if that is what you mean).

Does the system "see" the right amount of memory with each of the modules?
i.e. with just the 512mb module installed, does XP report 512mb?
...   with just a 1GB module, installed, does XP report 1GB?
...   with the 512mb and a 1GB, does XP report 1.5GB?
...   with the two 1GB modules installed, does XP report 2GB?

It reports each of the modules correctly when individually installed with no other.  The problem seems to be when I try and install more than 2gb (in aggregate).  The Asus motherboard permits up to 3gb, but it is not recognising more than 2gb.

I've seen that before. I've installed 4GB of ram in single channel configuration, and saw 3.25GB (which was expected because it was 32-bit windows). I put it into a dual channel configutation, and reported memory went down to 3GB.

I used to think years ago that, some day, they will actually build a memory controller that can handle any combination. I guess we will have faster-than-light travel before that day ever comes.

:)

Please see attached image as an example of the problem - it shows banks 1,2,3 with RAM but not reporting the full capacity.
memory.JPG

Ok, Ive managed to solve the problem.

Basically, the 3gb of RAM was not showing because as I increased the size of the graphics aperture size from 64mb (default) to 512mb thinking it would make the system faster.  Anyhow, reducing this back to default enables the system to recognise 3gb again.

Before I close this question, given that now each of the 1gb modules are working, but one of them is a slightly slower speed i.e. 2x 2.5T and 1x 3.0T, what is the order in which they should be placed in the three memory banks?

Thanks.

The order won't matter -- as I noted earlier, the system will automatically "... operate at the lowest speed and highest CAS latency that's common ..."