How much data are you backing up, over 2TB?
Are all the servers "local" (at the same location) or at different locations?
What application/method are you using for backup?
Can you colo equipment at the DC or only lease what they have?
1. Directly Attached Storage - plug into an existing or mid-spec server: HP MSA 60/70 or Dell PowerVault MD series.
2. A small SAN device such as HP's MSA10xx <- Much more expensive but more potential for growth (starts at 4TB)
There is plenty of I/O in each of the above, spec your head-end to your backup software & load estimates (simul backup/restores/client requests). 2xXeon3.0 + 2GB RAM should suffice (fast local disks for the OS/apps and scratch).
Well you have more room to grow with a DAS disk shelf, and you can use an existing server. The SAN scales to 48TB but as I mentioned is the most expensive option (but you can also task a slice for other use such as near line storage which will increase it's ROI).
You can always put a 3ware 9550 or similar into a case and get 16 SATA channels, which will give you 4.8TB of raw space, but I guessed you had already thought of that option and were looking for alternatives.
With that many disks you need an excellent chassis and DC cooling.