Have an ageing but stable homebuilt P4 set up. Have 256MB vid card installed. Ram is 2x256 16 800 45ns. Have obtained a pair of 256 8 800 40ns, will these work or should I return them unopened.

You've left out the most pertinent information.

What motherboard do you have?

What does your motherboard manual say about ram configuration?

Without that we're guessing.

I used to have a signature but it disappeared and I just couldn't be bothered writing another so please feel free to ingore this.

Thanks for responding. Didn't build it myself and havn't got a motherboard manual, can I find the info you need on the board?

You can either open the case & look for a model number, or try running CPUZ:

http://www.cpuid.com/cpuz.php

Thankyou,
Tried CPUZ, doesnt give name of mboard. Chipset: ATIRS300/RS300M rev 02
Southbridge: ATI SB200
BIOS: Samsung ver. C50A.xxxxxxxx.xxxx.HJV
Date: 08/19/2005
Socket 479 mPGA
Do I still need more info?

"Chipset: ATIRS300/RS300M rev 02
Southbridge: ATI SB200"

Going by that your mboard is probably no more than about 4 years old.

Try this:

Start - Run - type: msinfo32 (click OK or press Enter)
Look at the listings on the right side.
System Manufacturer -
System Model -

Or - look on the first screen as you boot the computer for the model number and maybe the maker of the mboard - press the Pause key to read it - any key but Pause to continue, or Enter.

Or - Go here, download BIOS AGENT.
Run BIOS AGENT.
Tell us everything Bios Agent finds, and include any dashes, etc.
- here's the link that downloads Bios Agent
http://download.esupport.com/biosag...
....

It is NOT a good idea to buy ram these days without first checking out whether it will work in your mboard for sure, especially for DDR ram.

See response 5 in this for some info about ram compatibilty, and some places where you can find out what will work in your mboard for sure:
http://www.computing.net/hardware/w...
Correction to that:
Mushkin www.mushkin.com

Thanks.
The computer is 5 years old this month. Running msinfo 32 i get:
System manufacturer: Samsung Electronics
system model: P29/28/26
The memory currently fitted is:
2x 256 16 800 45ns
I ordered another pair from Maplins but after 3 failed attempts! they sent:
2x 256 8 800 40ns. What i need to know is will these work as a pair with the other 2 to give me 1024MB or do I need to send them back. If I open them they wont take them back.
Thankyou all for your excellent service.

"System manufacturer: Samsung Electronics
system model: P29/28/26"

P29/28/26 may not be specific enouugh.
Apparently you have a Samsung P28, or P29, or P26 notebook computer.
Look on the outside of the Notebook for it's specific model munber, probably on a label on the bottom of it.
e.g. NB-P29xxxxx
If there is no label, look in your bios Setup for the model number, or try Bios Agent.

There is ram listed for Samsung P28 and P29 on the Kingston site - they both list the same ram modules.
There is ram listed for Samsung P28 and P29 on the crucial site - they DO NOT list the same modules.
Huh??
So it's important to know which model you have.

I found no ram listings for P26.
......

You can get a manual, drivers, etc. on the Global Samsung web site.
Search for P28 or P29, whichever applies.

"I ordered another pair from Maplins but after 3 failed attempts! they sent:
2x 256 8 800 40ns. What i need to know is will these work as a pair with the other 2 to give me 1024MB or do I need to send them back. If I open them they wont take them back."

If you can identify who actually made the ram, and can see the manufacturer's ID number on the ram, without opening up the package, you can probably look up whether it will work in your model.
If you can't do that, since they don't allow returns, don't take a chance - send them back. There is no way of confirming whether the ram will work for sure on the Maplin's web site that I can find.

Places that sell memory as well as other computer pieces are not ram experts in most cases. If they don't have some way of confirming whether the ram you were thinking of buying will work for sure in your computer on their web site, and if you can't look up whether the ram they sell will work elsewhere on memory manufacturer's or distributors web sites that DO have a means of looking up whether the ram will work for sure by using quoted brand and ram module ID strings, shop elsewhere.

According to the info on the crucial web site, P28 and P29 have only 2 ram slots. If you want 1gb total ram or more, you need 2 512's, or 2 1gb's, and you take out what ram you already have.

Oops.
"If you can't do that, since they don't allow" opened ram "returns, don't take a chance - send them back."

By the way, you must remove the main battery and the AC adapter whenever you remove or install ram.

"If you want 1gb total ram or more, you need 2 512's, or 2 1gb's, and you take out what ram you already have."
Or a 256mb and a 1gb, or a 512mb and a 1gb, etc.
Your computer does not need to have matched pairs in a set, and those are not applicable or available.