Requiring one element without attributes XSD

I'm just getting started with XSD trying to validate an XML configuration file I've been using. Each configuration specifies a default configuration for a server, which can be overridden on a per-server basis.

This is the default:

 <server>
  <cpu>65</cpu>
  // Other configuration
 </server>

This is an (optional) override:

 <server key="2">
  <cpu>55</cpu>
 </server>

I'm not sure how to build the XSD to support 1 required element without attributes, and 0-n elements with attributes given that they have the same name. With different names, it's a much easier matter, but that seems messy given that the elements are otherwise identical.

Edit:

To address @kjhughes's request, here's the simplified document structure:

<configuration>
  <target>Production</target>
  <responsible>email@email.com</responsible>
  <server>
    <cpu>65</cpu>
  </server>
  <server key="2">
    <cpu>55</cpu>
  </server>
</configuration>