Is there a possibility that much of the end-user data could be (substantially) duplicative? If so then expand your backup solution to include a review of data deduplication product offerings (there are many).
* an example of a client software solution to compare with Retrospect and NTI would be the EMC Avamar product.
* appliance based deduplication is available from vendors like SilverPeak, Expand Networks, Data Domain, Juniper, and others to replicate data between sites over a WAN.
* sophisticated NAS vendors also offer volume based data deduplication and that would be a modern variant of snapshots when used with sequential backups.
For commercial storage offerings an inexpensive path would be the unlimited Continuous Data Protection equivalent services from Carbonite and Mozy (EMC's consumer branding of Avamar). Annual fees per seat is ~$50/year with _unlimited_ client storage.
At a high level a helpful planning resource from the Virtualization world is the downloadable VMware book : "A Practical Guide to Business Continuity & Disaster Recovery (with VMware Infrastructure)", which you may find as a handy planning reference. VMware Item # VMB-BCDR-ENG-Q308-001.