vRAM capacity will be calculated based on the number of licenses loadded into the vCenter Server to which you connected. Make sure that all your VMware vSphere/ESXI licenses are added into the VMware vCenter Server to get better insight into the vRAM capacity you will have if you upgrade to VMware ESXi 5.
Follow below steps to install VMware vSphere Licensing Advisor,
Download VMware vSphere Licensing Advisor
vSphere Licensing Advisor installation is simple & straight forward. Download & Launch vSphere Licensing Advisor setup
Accept license agreement & continue
Select destination & click next,
Tick launch vSphere Licensing Advisor & finish the setup.
Enter your Vmware vCenter Server details,