upgrade memory on C640 lattitude doesnt work

I originally had two 128 mb cards in my lattitude C640.  I purchased a 256mb from Dell and took out one of the 128 cards.  When I view system info on my computer it still says I have only 256  mb.  It should read 512 (128 + 256).  I am sure it is installed as it clicked into place.  Any clue on why it isnt working. 

System info:

 

Available Memory 25.30 %Page File Size 1792.4MbAvailable Page File 18.11 %Virtual Memory 2047.8MbAvailable Virtual Memory 98.03 %Memory Slot 1 [DIMM_A] 128MbMemory Slot 2 [DIMM_B] 128Mb