new installed memory not recognized-Dell Inspiron 8500

I've searched for a while, but I couldn't find the answer to this problem. I have a Dell Inspiron 8500. I decided recently that I wanted to upgrade my RAM. I originally had 2x256 MB, so I decided to buy a 1 GB card from Dell. Dell's site said that the maximum memory that I could have was 2 GB, so I'm not exceeding the maximum memory.

Anyway, I took out one of my 256 MB cards and replaced it with the 1 GB card. I've looked at the Bios and it now says that I have 1280 MB of memory installed, so it looks like the hardware is recognizing the new memory. However, when I get into Windows XP SP2, I right click "My Computer" and click "Properties" and the system says that I have only 508 MB of RAM. Then if I click Ctrl+Alt+Del and look under the performance tab, it says that my physical memory is only 519726 KB. So it looks like Windows XP is still only recognizing 512 MB of RAM and not 1280 MB.

I have no clue what to do. I've tried switching slots between the 1 GB and 256 MB cards and got the same result. I've also updated my Bios to the newest version and have just tried using only the 1 GB card. When I do that, the Bios says 1024 MB of RAM, but Windows XP still says the same thing. Can anybody help me with this problem? I just don't understand why Windows XP can't recognize the increase in RAM. Thanks so much!

Do you mean just try using only the 1GB card? I did try that and my bios says 1024 MB of ram, but again windows xp says the same thing - 508 MB after going to "My Computer"... and 519726 KB when looking at the performance tab under Ctrl+Alt+Del.

It looks like the hardware is working properly because the bios is being updated correctly. I just can't understand why windows xp isn't consistent with the bios.

you do NOT need matched dims unless its across dual or quad processors
I currently have the following:

sony vio - 1 512 and 1 265 (laptop)
sony vio - 1gb and 1 256 (laptop)
e-machine - 1gb and 1 256
hp - 1 gb and 1 256
dell - 1gb, 512 and 1gb, 512

I had the same problem with my dell. I had to go into the bios and tell it to find the ram. Its been a while but i think i just went to bios/memory and there was something like refresh or reconfigure.