system can not recognize my 1.5G memory by default

["Dan Liu" <miraclecome@gmail.com>]

I have 1.5G memory,but my etch Debian can only recognize 1G.
$ free

             total       used       free     shared    buffers     cached
Mem:        906792     274832     631960          0      10904     149100
-/+ buffers/cache:     114828     791964
Swap:      1004020          0    1004020

So I try to change this bad suituation.I change my /boot/config-2.6.18-5-486 file.
Previous:
CONFIG_NOHIGHMEM=y
# CONFIG_HIGHMEM4G is not set
# CONFIG_HIGHMEM64G is not set

Now:
#CONFIG_NOHIGHMEM is not set
CONFIG_HIGHMEM4G=y
# CONFIG_HIGHMEM64G is not set

Rebooting my computer.Memory still can only recognize 1G