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

[Stainforth, Matthew (SD/DS)]

> >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

Use a PAE kernel or the 64 bit version of RHEL.

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

64 bit OS will support 64 bit processes (i.e. your oracle instances), which can address more than 4GB of memory each.  PAE will only support 32 bit processes, which can only see 4GB of memory, but the system itself will recognize all 16GB.

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

I'm not sure that's a supported upgrade path.  Your best bet will be a fresh install of 64 bit RHEL and migrate to it.