This post is pretty much a bunch of links that I have found that I am using to get up to speed with ZenServer. I've got a new virtualization project just around the corner and Vmware is no longer a good fit for us.
The link below is from Citrix.com and compares the various features in the different versions of XenServer.
http://www.citrix.com/English/ps2/products/subfeature.asp?contentID=2300456
And the link below explains XenServer licensing. There are three licensed versions of XenServer, and of course a free version. The specific version that I am looking at is Enterprise, which only lacks a few features available in Platinum. Street price for Enterprise is $2500 per server.
http://www.citrix.com/English/ps2/products/subfeature.asp?contentID=2313292
If you are interested in test driving a licensed version of XenServer, you can use the link below to register for a 45 day trial.
http://deliver.citrix.com/go/citrix/ceXenServerTrial
Last but not least, the link below is to Citrix's documentation portal.
http://support.citrix.com/productdocs/
Now just need to find the minimum and maximums docs and I should be good to go.