The BIOS tells the kernel how much memory is available, although the kernel also performs its own checks. What does the BIOS say?
Check /var/log/messages for the memory initialization line right after boot. Should be something like this:
Oct 29 17:04:38 smallpond kernel: [ 0.000000] Memory: 2015320k/2061952k available (5470k kernel code, 46180k reserved, 2611k data, 624k init, 1156744k highmem)
I've used Linux servers with 256GB of physical memory, so I know its not a kernel limitation. I don't think Redhat adds any kind of limit.