According to the hardware (Dell R710) I assume this is going to be a production system!? In this case you may consider the Essentials Kit - as mentioned by vmroyale - anyway. This not only allows to use unlimited (up to the supported limit) memory, but also enables the vSphere API to be able to use VM based backup applications.
Another option - if you still want to use a free version - is to e.g. use ESXi 4.1, which allows 256GB, which is limited to physical 6 core CPUs though.
André