Dual-Channel, Slower?

You cannot mix ram. It has to be all the dimms the same or none at all for everything Dual Channel to work.

It will default to the slowest stick and dump the rest.

I use Quad Channel Module kits because they come in matched sets of 4 and run very well indeed together.

But if you compare the speeds with only 2 sticks to 4 sticks (dual-chan), nothing changes except the added memory...

That isn't true. You can mix ram all you want and I really have no clue why so many people try to spread this misinformation. Sometimes it won't work, but most of the time it will. Typically if you mix ram it will operate at the slowest common speed. This is why if you buy more ram you should always buy more of what is actually in your computer. I will agree that for the best chance at optimal performance you should buy identical RAM from the same manufacturer. I used to run 2.5GB myself and never had a problem.

1) when you are mixing the ram the bios is trying to set the correct timing. Obviously it doesn't like it and downgrades the speed for reliable performance. Running in dual channel is fixing this problem for you. You may be able to manually fix it, but why would you want to run in single channel mode.

2) how many sides the memory chips are on has nothing to do with DIMM/SIMM. I have a few single sided DIMMs myself.

3) The only difference in speeds is between dual and single channel operation. In the single channel operation you have a slower clock rate, but a faster latency (less waiting time for the memory to respond to a command). However, your latency is only one clock cycle faster (1/167MHz) for tRAS and tRC. The higher speed and two channels is more important than saving a single clock cycle from being wasted.

4) memory runs fastest when you only have a single DIMM per channel, so option 1 should be the fastest followed by 2, and then 3.

You can use a tool like this one - http://www.passmark.com/products/pt_advmem.htm - to confirm which is the fastest setup.

"memory runs fastest when you only have a single DIMM per channel"

Don't the colors represent the channels?  

And are you sure only using the 2 1Gb sticks will be better? The specs seem the same except for the added memory.

The difference is negligible. Just run the 2.5GB.

What is the make and model of both sets of DIMMs?

At least post a copy of the SPD table, one for the 1GB DIMM and one for the 256MB DIMM.

A picture of the SPD tab page from CPU-Z for a 1GB module and for a 256MB module would be helpful.

Here is the SPD for each:

1 GB:


256 MB:

They are both the same image file.

Oops.

I fixed my post.

Questions:

1. When I run it in single-channel, the bios says my RAM is PC2700 rather than PC3200. In CPU-Z all my RAM sticks are PC3200... Why does it downgrade?

The BIOS has no clue what to do with the size mismatch for a channel pair so it'll choose a lower setting (i.e. slower memory clock and maybe looser timings) that hopefully allows the system to POST.


2. The 256Mb sticks are only one-sided, so I'm guessing they are SIMMs? Could this pose a problem?

SIMMs are either 30-pin or 72-pin so you don't have either one of those.

The ASUS A8N-LA motherboard only supports 64-bit 184-pin DDR1 DIMMs.


3. Could you explain to me the differing speeds shown in the pictures? Some of them seem worse in dual-channel?

Your single channel mode operation picture shows that the memory is operating at the slower 166.67 MHz memory clock speed. Slower memory clock speeds can handle tighter memory timings.

The dual channel operating mode pictures shows that the memory is operating at the faster 200 MHz memory clock speed.


4. I'm currently running in dual-channel with all 4 sticks. Is this my best option? Will this give me the most speed? I have a suspicion that my PC was faster with only 2 sticks...

If you can manually set the memory timings, I don't know if that's available with your OEM motherboard, I would suggest using only the two 1GB DIMMs in the blue memory sockets and set the Command Rate to 1T to get the best performance.