Trying to fit new 1gb memory stick in dell optiplex sx 270 but the system will not acccept it and not boot up error codes of ab yellow but cd green all it does is beep when fitted yet i refit the 256mb stick and it boots up with no problem and the 1gb stick works fine in my other pc

What Operating System are you running - if Windows what version XP, vista, 7? When is it shutting down, after you get into Windows (or other OS) or when trying to boot up? Are there any error messages? New machine or old machine - if old has anything changed? How long does the system stay working before it shutsdown? Is it getting too hot, fans blocked etc?
Lets start by working out if it is the hardware (Dell and devices connected) or the software (Windows etc) It is difficult to know without more details of the problem but here are my best suggestions
Try safe mode - running windows try in safe mode (F8 when starting windows), check error logs
Test the hardware - I would start by disconnecting all external devices, USB disk and other devices. If possible also remove any extra boards you have added to the machine. Even the mouse, USB sticks, anything plugged in (but leave the keyboard). Try this, do you get the same problem.
Test the OS - If you have a live CD such as Ubuntu (can be downloaded from here: http://www.ubuntu.com/desktop/get-ubuntu/download ) then try booting the system from this and see if you still get a problem
Can you run a virus checker on it, how long does the machine stay running. You can try downloading from here from another machine - http://www.avg.com/us-en/avg-rescue-cd-download and making a CD, run the rescue anti virus.

Hi yes it can speed up, first you need check in your manual what type of memory your pc is use, then buy stick with same specification witch is in otherwise it can slow your pc. Sometime it better replace it for one stick 1Gb if it help please rate it regards

Hello

The LED code you described indicates a memory failure. Remove and re-install all of your memory. If this does not fix the issue, remove all memory and insert one stick at a time until the unit turns on. Replace faulty memory.

Here is the LED Code guide from Dell...

http://support.dell.com/support/edocs/systems/opgx270/en/ug/solve.htm#1235408

the 6 beep codes will be in a specific timing i.e. two beeps, a space 2 more beeps, and another space and 2 more beeps.

Looks like it may be a memory problem according to the lights.

Possible memory failure

Reseat all DIMMs and restart the system to retest.

Here is the link to the beep codes and diagnostic lights for your dell optiplex gx 150 HERE

The Beep codes are in Table 2 and the diagnostic lights are in Table 3.

Desktop and Minitower GX270s can go 4GB RAM
http://www.crucial.com/store/listparts.aspx?model=OptiPlex%20GX270%20Series%20(Desktop%20and%20Mini-Tower)
The small form factor chassis can handle 2 GB RAM
http://www.crucial.com/store/listparts.aspx?model=OptiPlex%20GX270%20Series%20(Small%20Form%20Factor)

You are limited in the choices for video card The Optiplex GX270 only has a 160W power supply Not only that but because of the slim case, you're most likely limited to low profile video cards. That means the choices will be extremely limited & not very good. Even after the upgrade, you'll most likely have to stick with older games. follow this link to buy low profile video cards http://www.directron.com/lowprofile.html

That is a memory failure 1 or more sticks of memory are malfunctioning, here is a link with all the information about the diagnostic lights as well as a beep code list

http://www.austincc.edu/cloud/manuals/Dell/GX260/solvprob.htm#1167139

Open up the case and remove any add in cards and memory modules. Try to start up by adding these back one at a time until the unit stops booting normally. The light combination is not one recognised in the user manual so I am guessing you have a non-descript fault with one of your components. Be careful when you open the case as to not disturb the ribbon to the motherboard which controls the power, I have had a very long experience trying to get one of these reconnected!

You may need to install memory in matching pairs. If the new memory is a single 1Gb module and you didn't already have a similar one they may not work together. Try removing the original module(s) and see if it makes a difference.