RHEL 5.1 (32 bit) shows 3GB RAM though there's 16GB physical

[sunhux G]

Hi

>From /proc/meminfo  &  top & dmesg, our 64-bit hardware
showed only a total of 3GB RAM.

Physically 16GB RAM were inserted.

Q1:
Is top/dmesg/meminfo giving the right info or how to get
RHEL 5.1 to fully utilize the 16GB

Q2:
Is 32 or 64 bit RHEL 5.x better at utilizing memory?
The box hosts 7 Oracle instances


Is there a lot of effort involved to upgrade from 32 bit
to 64 bit RHEL 5.1?

Tks
U