I have a DELL optiplex gx270. The unit has four slot to put ram in. Two of the slots have 512 MB ram chips and two are empty. for some reason the unit only recognizes 256MB of the 1024 installed. I initially THOUGHT that MAYBE the chips were bad, so I replaced them with one from a different machine (a single 1GB chip). The computer still only recognized 256MB of the 1024MB installed. Any input would be appreciated.
I just realized someone else started the same topic right before I did. The set that are in the unit are 512MB DDR 400MHz cl3 that came originally in the machine.
Does this help ...or this?

http://www.crucial.com/store/listparts.aspx?model=OptiPlex%20GX270%20Series%20(Desktop%20and%20Mini-Tower)&pl=Dell&cat=RAM

You may have to flash the BIOS in order for the system to see all the RAM...After taking a closer look, I noticed when I booted the computer it said operating system install mode (I just put window XP on this machine). The system limits the amount of ram to 256MB when the the OS install mode is ACTIVATED. I turned it off and it recognized both 512MB ram chips again.

Thank you for your reply though.