About the things specifications for Inspiron531s don't tell us.

  Specifications for this machine tell us its memory is
512MB or 1GB. This machine I selected indeed has two
512MB memories. Picture A shows two preinstalled 512MB
memories work normally. The software package for this
verification is Memtest86 (http://www.memtest86.com,
Download-Memtest86 v3.4a ISO image(zip)).

                       Picture A


  At the same time, there are well-known things in
the things specifications for this machine don't
tell us. Its memory capacity is above 1GB and can
be 2GB. Picture B shows two 2GB memories work
normally.

                       Picture B


  Memories used in this verification are DDR2
PC2-6400 2GB x2 (Pulsar DCDDR2-4GB-800).