Adding RAM to Optiplex GX280

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Hi all

Can anyone advise on the best configuration for increasing RAM in Dell
Optiplex GX280s please?
The slots are arranged in 2 separate pairs.
As supplied, one slot in each pair has 256Mb.
As part of a general upgrade I have been recommended to upgrade RAM - the
PCs are running XP.

So can I add 1 x 512Mb module alongside one of the existing, or do I have to
add 2 modules to create 2 balanced pairs?

The current memory modules are labelled 256Mb    1Rx16



Re: Adding RAM to Optiplex GX280

TheScullster wrote:
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The biggest sticks Crucial offers are 1GB each. Most of the time, they
use the manufacturer (Dell) info, when listing memory.

DDR2 PC2-5300, CL=5, Unbuffered, NON-ECC, DDR2-667, $32 for 2x1GB. (Desktop%20and%20Mini-tower%20Models)

At that price, I'd just pull the original memory and replace it.
You could install 4GB for $64, using four matched sticks.
Keep the old sticks for emergencies - if the RAM ever fails,
it wouldn't hurt to have some to test with. The resale value
of the existing RAM is perilously close to zero.

 From an Ebay advert for a Optiplex GX280 motherboard:


* Intel 915 Express ChipSet
* Socket LGA775 Supporting Pentium 4, Celeron
* Hyper-Threading Technology Support
* 533/800 MHz system bus
* 4 Dual Channel DIMM Support for DDR2-400/533
* 4 GB MAX system memory
* 1 16x PCI-Express Video Slot
* Integrated Intel 900 Graphics Accelerator
* Integrated 10/100/1000 Lan
* Integrated ADI Sound
* 1 Serial ATA Hard Disk Connectors
* One ATA-66/100/133 IDE Hard Disk Connector
* One Floppy Connector
* 6 USB 2.0 Ports
* Audio Ports for line-in, line-out, and microphone
* 1 Serial/1 Parallel Port

Notice that Crucial is selling memory which is faster
than the computer specification. That is because a
faster memory can operate at slower speed. The DDR2-667 memory
can be run at DDR2-533 or DDR2-400. (I use DDR2-800 memory
in my DDR2-533 system.)

For your reading pleasure, there is this doc from Intel.
This is the 915 memory guide. In some of their later
guides, they messed up some of the drawings, and never
bothered to fix them. So if one of the drawings looks goofy,
it is because someone doesn't care to fix it.

I also had a look at the 915 datasheet from Intel, and this
is a summary.

1) Matched pairs work best. The 915 supports dual channel
    (and your chipset is likely to be the 915G).
2) Mismatched *quantities* of memory, run in virtual single
    channel mode. So there is less memory bandwidth that way.

    512MB on one channel, 256MB on the other, is a quantity
    mismatch. The system behaves then, as if it is a 768MB
    single channel system.

    If you had 2x256MB on one channel, and 512MB on the other
    channel, that would be a match of quantity, and would run
    dual channel. The end result in that case is 1024MB of RAM,
    running dual channel. Even though three sticks of RAM are

3) Apparently, the largest module size for the 915, is 2GB.
    So you could install 2x2GB. The chipset has a 4GB limit,
    and with 2x2GB, there would be no point in installing more
    memory, as it would be ignored. Installing 4x1GB
    works just as well.

    With the advent of cheaper 128MB chips, it is possible now
    when you buy a 1GB module, to get single sided RAM (i.e.
    1Rx8). That should work in this system, so unlike some
    VIA chipset based systems, there shouldn't be any surprises
    when upgrading.

When you install 4GB of memory, and use WinXP 32 bit, you
don't get all the memory reported as "free". Some of the
memory is ignored, leaving address space for other resources
in the computer. You might see 3.2GB or so reported as free.
If you wanted, you could install 2x1GB + 2x512MB, so that
all the installed RAM would get used. (It would report 3.0GB
free in that case.) But for $64, I'm not sure the distinction
is worth the bother. Buying the 3GB configuration would be $56.

If you're tight for cash, 2x1GB might well be plenty.

The thing is, when you have 4GB, you'd be surprised how
difficult it is, to get a single program to use nearly
all of it. I tried a few experiments once, and didn't
succeed with the programs I've got. The programs were
stubborn, and stopped at around 1600MB or so. For more
info, articles like this go over some of the details.


Re: Adding RAM to Optiplex GX280

"Paul" wrote


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Thanks for such a comprehensive response Paul that's great.
Can you just clarify the term channel for me please?
Is each "pair of RAM slots" on the motherboard 1 channel?
So you shouldn't put different sized modules side-by-side, but you can put
different sizes in providing the pairs match?

Thanks again


Re: Adding RAM to Optiplex GX280

TheScullster wrote:

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In this picture, (1) and (3) are on the same channel. (2) and (4) are
on the other channel. For dual channel operation (1)+(3) = (2)+(4) in
quantity of bytes, so on this chipset, the quantity is what should match.

This would be an example of dual channel.

    (1) 256MB
    (3) 256MB

    (2) 512MB

With matched pairs, it is obvious the totals are equal as well.

    (1) 1024MB

    (2) 1024MB

Crucial does not sell the following config for your computer, but
according to the available documentation, this should work also.
If you install 4x2GB, half of the memory will be ignored.

    (1) 2048MB

    (2) 2048MB

Sometimes, the problem with 2GB sticks, is the BIOS doesn't have
stable settings for them. That is the problem with my
motherboard. The hardware supports 2x2GB, but the BIOS does
a poor job of setting the parameters for it. So I have to live
with 2x1GB. There is no way to predict how the 2x2GB would
turn out. But when the Dell product was released, they
probably had access to 1GB sticks to test with.

Don't forget to test with memtest86+ from, immediately
after you install the new memory. Don't boot into Windows, until
you've run a couple test passes, and verified all installed memory
is happy. If there are errors reported, you can try testing the sticks
one at a time.


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