You need to install a PAE kernel. Open Synaptic package manager > click Reload > click Mark All Upgrades > click Apply > click Apply in the confirmation window. When that has completed, click Search > enter "kernel" (without the quotes) into the search field > via the drop down menu, change Description and Name to Name > click Search. Look for and mark to install either the PAE or the BFS.PAE kernel > Apply > Apply >reboot. Be patient during the first boot up. All your modules and drivers need to be built against the new kernel. It will take time.