PST files should probably be considered caches of data on already on exchange/imap servers. Generally, caches should be ignored, like temporary files. If using exchange, you can backup everything at once, using something like veritas on the exchange server. If using an imap server, it should be simpler again.
Users' Windows Documents (most of their profiles actually) can be stored centrally too, and again, can be backed up from a central location. You could do this using a fileshare on the server, or using a NAS/SAN box.
You also have the option of replicating windows active directory/exchange servers (which gives you failover/scaling), and then backing up the replicated box to avoid slowing down the main server.
Oh, and stay well away from solutions like VMWare: especially when they're solutions in search of problems, or like in this case, the wrong solutions to problems. VMWare will sell you this whole song and dance about how using their stuff can consolidate servers and save you money. What they don't tell you is that VMWare means big performance hits, and to even it out, you need to invest in really good disks subsystems (like a SAN solution) etc.
Xen is better, but I wouldn't use virtual machines except in a few specific use cases:
building virtual dedicated servers, and having clients rent them at about 1/4 of the price of the actual physical hardware. For internal stuff, that just needs to be functionally/organisationally isolated rather than secure, something like chroot or lxc is much better.
trying things out on a small scale before buying real hardware
testing cross-platform development work.
running multiple OS's on my own desktop machine for app compatibility or infrequent user support
Edit: Given what you've added about being a struggling company etc., I would suggest hiring in a unix or windows specialist, and having them set you up a mailserver with tried and PROVEN backups, along with a simple way to monitor the backups that you won't start ignoring when 99% of the messages all say "Last night's backup completed successfully."
For the files, buy two good NAS boxes that support remote backups, move everyone's profiles to one, and backups going to the other. Or, rent an off-site backup service. Remember that backups of just yesterday is no good. You need a backup of yesterday, a backup of the day before, the week before, the month before, six months before, and preferably each year back as far as tax/industry records require.
Bear in mind that off-site backups are always recommended --- a company's data represents all its work, so even if the building burns down, it's the DATA backups that really matter. And use that argument if you need to fight for funding on this.