OS does not see all my RAM

[robin at bullseye.tv (Robin Mordasiewicz)]

My BIOS shows all my RAM, but when I boot up linux it does not see all the 
RAM

I have two one GIG sticks of RAM, which show up in my bios, but when 
typing free, I see that it only shows 256Megs.

Is there a way to specify the amount of RAM if linux cannot detect this, 
or how can I fix/tweak the OS to recognize the amount of RAM

my OS is centos-4-x86_64 on a DELL 1850

[root@smtcorav02 ~ ]# free
              total       used       free     shared    buffers     cached
Mem:        251360     220136      31224          0       3932      64748
-/+ buffers/cache:     151456      99904
Swap:       524280          0     524280


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