[CentOS] Not seeing all memory in CentOS 5.1 x86_64

I'm running CentOS 5.1 with all updates, and the xen kernel.  For some 
reason the OS is not seeing the full amount of ram. 

#uname -a
Linux CentOS-VM-A 2.6.18-53.1.21.el5xen #1 SMP Tue May 20 10:03:27 EDT 
2008 x86_64 x86_64 x86_64 GNU/Linux

# free
             total       used       free     shared    buffers     cached
Mem:       6104064    3445136    2658928          0    1412236    1515032
-/+ buffers/cache:     517868    5586196
Swap:      2031608          0    2031608


On another box with identical hardware, I see
# free
             total       used       free     shared    buffers     cached
Mem:       5038080    1818980    3219100          0     145208    1148608
-/+ buffers/cache:     525164    4512916
Swap:      2031608        152    2031456


Both of these boxes have 8GB of ram.  Is there a reason I'm not seeing 
all of it? 

Russ