[CentOS] OS does not see all my RAM

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