<p>I'm looking to run a Jabber server on a Windows 2003 server(web farm) and like some practical advice from anyone who has run a live environment with ~500 concurrent users.</p>

<p>Criteria for comment:</p>

<ul>
<li>Performance</li>
<li>Capacity (ie ~number of concurrent users)</li>
<li>Stability</li>
</ul>

I think you're going to need to be a bit more explicit - you looking for server configurations, or software e.g. Jabber Server?
If you're thinking Jabber server,
EJabberD
is probably the most stable, flexible, capable of being clustered etc.
Really useful comparison of Open Source servers here...
http://www.saint-andre.com/jabber/jsc/