I need your help, New user

I am planning to have a vSphere complete system for my office use.

I will have one vCenter (Dell R320) and two ESXi servers (Dell R510) for clustering, and I will have SAN storage (Dell MD3200)

My Question is: regarding the RAM, I have 4GB for the vCenter server, and 8 GB on each ESXi Server.
I will use version 5.1
is is enough or I need more RAM especially for vCenter?.