I'm not sure on how licensing for Xenserver works, but I think you can use it for free, and the only paid stuff is the essentials. XS 5.5 installs its own operating system, which obviously reduces overhead and increases speed, so you may want to look into it.
For that backup box, you will want to look into RAID-10 or RAID-6, as well as a good dual core or even quad, depending on what operations you will be doing on the backup box, such as encrypting and decrypting, and how often you serve files from the box, or upload to the box.
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cristibighea is correct that you can use xenserver 5.5 for free in a production environment. The free version is actually pretty complete. Really all you're missing in the free version is "StorageLink" for SAN Management (if you have a netapp or equalogic appliance) and High Availability/failover. You can always go back and purchase the licenses at a later time as your needs change, though. You may have to build some of your own tools if you're going to use XenServer for hosting, however, so you better know some system-side programming languages or have some money to spend on a decent developer.
As for backups, I have two iSCSI SAN's and I mount a production partition and a backup partition (one from each SAN) of equal size onto each vm and simply run an rsync shell script once daily. This may be wasteful depending on your needs, but it works well for me.
Last edited by e-Sensibility; 10-05-2009 at 12:43 PM.