I recently got my hands on the latest version of Citrix EdgeSight for Load Testing (3.5) and got some time to use it to determing performance differences under similar loads for various operating system versions.  For those that have not seen or heard of ESLT.  ESLT is an EdgeSight product that allows you to record user sessions in ICA,Web Interface and XML interface sessions and replay them using many users simultaneously against a target machine.<!--more-->

This gave us a valuable tool for producing consistant loads and measuring performance on each.  For our test we used a load of 40 users over a 10 minute period doing typical operations in the system then logging out.  With ESLT's ability to capture performance monitor (perfmon) counters in it's interface then providing reports it made crunching the data even easier.  Being untrusting of new to me software I went to the servers and captured the perfmon information there as well and analyzed the logs to find the results comparable (which it should be since it is from the same source).  I would recommend working with ESLT when trying to gather critical performance data to make educated decisions about server sizing and performance issue remediation.  Here are some uses for ESLT (straight from Citrix themselves).
<ul>
	<li><strong>Deliver tried and tested application virtualization environments</strong> - Simulate thousands of users to identify system bottlenecks or application instability before systems go live</li>
	<li><strong>Reduce costs associated with change management and application regression testing</strong> - Reusable scripts simplify before and after comparison following application patches or operating system updates</li>
	<li><strong>Assess new hardware such as 64bit servers</strong> - Understand the capcity limits of different hardware types to make informed purchasing decisions</li>
	<li><strong>Perform branch office application performance testing</strong> - Create real traffic from remote sites ensuring bandwidth and server resources are available to deliver the best user experience</li>
</ul>
To add on to the stock information from Citrix I would suggest ESLT for comparing loads against several server versions and operating systems.  Comparing virtual to physical server performance.  Comparing virtualized application performance to their native counterparts.

Add in VMWare's AppSpeed to continually monitor multi-tiered application performance after the environment has been built and you have a well thought out, measured applications environment.