Detecting amount of memory?

Hi,

I trying to create a kickstart script on CentOS 5.5 that installs an RPM
depending on the amount of memory in the box. For example if the box has
>= 6GB memory then install RPM2 else install RPM1. With some Googling I
now have the following snippet:

%pre
memKB=$(free|grep Mem|awk '{print$2}')
memGB=$(echo "($memKB/1024)/1024"|bc)
BIGMEM=""
if [ "$memGB" -ge 6 ]; then
    BIGMEM="6GB"
fi

%post
yum install foobar$BIGMEM-1.0-1.rpm

The problem is that on a box with 6GB memory this snippet says it does
not have 6GB mem:

memKB = 6123716 KB
memGB = 5 GB

Anyone know what I am doing wrong?

Thanks!
Patrick