If your concern is storage I'm not sure what you think you'll gain from using an XServe.
The MacPros can handle more internal storage than the XServe, thanks to its four drive bays (vs. the XServe's three).
If you're planning to use the server as a network file store, that's fair enough, but don't think you can do live editing over the network - the throughput is going to be bottlenecked by your network and disk performance over the network is typically much less than local drive. It would be feasible to use it as an archive repository while running the current projects from the local disk, just don't try to edit HD video over the network.
A potentially better solution would be an XServe RAID rather than an XServe - the RAID offers up to 10TB of RAID protected storage which can be directly connected to your workstation, giving you the advantage of local file storage (viable for live editing) without the overhead of the network.
If you do opt for the server route bear in mind that all the other services (email, iCal, IM, etc.) are optional, and you can just ignore them until you're ready to deal with them.