2.6.4 Kernel + 4G of memory

[devnull at adc.idt.com]

Dear All, 

I would like to use the 2.6.4 kernel on my AMD64 box that has 

2 x Opteron 244 CPU's and 4G of memory. 

This shows 3.5G of memory when i use the 2.4.22-1.2129.nptlsmp kernel from 
Redhat, but only 3G when i compile 2.6.4 from sources.

(I am looking under /proc/meminfo)

I earlier stumbled on this problem when using the taroon kernels and was 
advised to update to 2.4.22-1.2129.nptlsmp

Is there anything in my 2.6.x kernel config that i should enable ? 

Thanks for your help, 

-- 
/dev/null

devnull at adc.idt.com