When adding to the original 4GB of RAM my laptop had I wanted to max it out to 8GB. Off to the shop, purchased and installed 2x 4GB cards and booted it up. The Bios recognises 8GB out of the box, but Fedora 15 does not with the default 32bit kernel. You need to install the following packages:
# install PAE kernel.
#
$ sudo yum install kernel-PAE kernel-PAE-devel
Once I did this, rebooted and you can see the memory is recognised:
# memory shown for 8GB after PAE kernel installed.
#
$ free
total used free
Mem: 8154348 5368672 2785676
$ grep MemTotal /proc/meminfo
MemTotal: 8154348 kB
Now my Java efforts are rocket fast on this machine!