Bios Memory count off

[rojo0550]

 I have a Gigabyte GA-8VM800PMD m/b with DDR 2 ram(512 MBx2),Intel
(Prescott)Celeron D,2.66 GHz cpu,LGDVD/CD RW drive,and Seagate
160GB SATA drive with Windows vista on it.
 The problem I have had for a few weeks now is that the memory count
in the BIOS screen shows that I only have 959 MB of RAM memory,when
about 3 weeks ago I had over 1 GB of RAM in the memory count!?!
 Performance wise,I have had some apps not responding,but otherwise the
computer has been fairly error free.
I was thinking that either the BIOS is corrupt,or that I might need to get
a good diagnostic utility to point where the fault lies?
Any helpful suggestions appreciated!!

[contrex]

Have you got some of the memory set up as shared memory for your display?

A good memory test utility is called Memtest86

http://www.memtest86.com/

I doubt if the BIOS is corrupt. I suspect that either you have forgotten that you are sharing video memory or one of your RAM modules is going flaky.