Desktop Optiplex Gx620 (Added Ram) Sees 3gb, only 2 usable?

[buscemis19]

My Computer is OPTIPLEX GX620. I decided to buy 2GB ram and add it to computer.
I was hoping to be at 4gb, but didn't realize the 4 sticks already in my computer were .5gb each.

Long story short. My computer has 3GB ram. 1gb, 1gb + .5gb + .5gb.

However, it says 3gb installed. (2 usable).

It doesn't make any sense to me why it would recognize the 3gb, but still only use 2.

I have windows 7, 64bit. not sure if that helps.

 

[Eximo]

Onboard graphics will consume system memory. Quite common really.

 

[buscemis19]

I'm not sure what that means. Does that mean I can't upgrade my Ram? Is it pointless to buy another 2 sticks of 1gb ram? Will that help?

 

[rgd1101]

http/www.dell.com/downloads/global/products/optix/en/spec_optix_gx520-gx620_en.pdf
check the existing 1GB and get the same 1GB