[CentOS] Memory: 2056396k/4194304k available

I have a centos 5 server running on olidata tegeo server with 2 opteron 1800 and 8GB ram.
I installed centos 32bit so I should be able to use 4GB. My system on boot recognize 4GB ram but allocate only 2GB.

Extract from dmesg:
Warning only 4GB will be used.
Use a PAE enabled kernel.
3200MB HIGHMEM available.
896MB LOWMEM available.
Memory: 2056396k/4194304k available (2043k kernel code, 39416k reserved, 846k data, 232k init, 1179584k highmem)

Why it suggest me to "Use a PAE enabled kernel."?


Thanks.

Giuliano.