Note that the minimum memory requirements is 2098MB from the initial 2048MB in ESX/ESXi 4.x.
Other related posts:
vSphere 5 vRAM Licensing
vSphere 5 ESXi 5 Installation
Upgrade vSphere vCenter 4.x to 5.0
Upgrade ESX/ESXi 4.x to ESXi 5.0 using Update Manager
Upgrade VMware Tools and virtual hardware with Update Manager