A good rule of thumb is that the machine should have enough CPU, RAM and diskspace to support the requirement of the applications running in each concurrent Citrix session. This varies depending many factors including the requirements of other applications running on the Citrix server, such as applications being tested by Certify, etc.
Citrix support website offers documentation for each version of their product as well as detailed information on system requirements. http://support.citrix.com/proddocs/topic/xenapp/ps-library-wrapper.html
A typical implementation that we've seen includes at least a dual core server, a minimum of 4 GB of RAM per concurrent user and a total RAM of at least 16 GB. Please also see the Citrix Xenapp Fundamentals blog for more tips on performance tuning. (http://www.citrixtools.net/Resources/Articles/articleType/ArticleView/articleId/5610/Windows-2008-R2-Remote-Desktop-and-XenApp-6-Tuning-Tips-Update.aspx)