Use dmidecode like this
dmidecode 2>&1 |grep -A17 -i "Memory Device" |egrep "Memory Device|Locator: PROC|Size" |grep -v "No Module Installed" |grep -A1 -B1 "Size:"
The "Locator:" line gives you the slot assignements as listed in the HP documentation, e.g. the HP ProLiant DL380 G7 page. Of course you can also look this up in the ILO GUI.