<p>Can anyone tell me what is the <em>memory usage overhead associated with PHP opcode cache</em>?  </p>

<p>I've seen a lot of reviews of <code>opcode cache</code> but all of them only concentrate on the performance increase. I have a small entry level <em>VPS and memory limits</em> are a concern for me. </p>

Most of the memory overhead will come from the opcode cache size. Each opcode cacher has their own default(e.g. 30MB for APC) which you can change through the config file.
Other than the cache size, the actual memory overhead of the cacher itself is negligible.