We do around 25TB (perhaps 30ish TB in a cycle) in a single full backup, and recently put in deduplication with Simpana 8. We've got a dedicated DDB Manager, and two data mover MA's. The DDB Manager is an R510 with X5660 (6 core CPUs). The R510 is great because it has support for a large number of spindles, and the H700 RAID controller is quite good.
We've got a few DDBs. Two are for primary storage policies, each has around half the total backup volume (around 12.5TB in each full backup). Each of those DDB is on a RAID10 set, with 300GB SAS2 6GB/s 15k disks. We also have a smaller DDB used for archiver data, this gets a LOT less data, so it sits on a RAID1 spindle. The operating system and page file also get their own RAID1 set.
We see the performance of this is way above anything we even approach needed. The disk queues never exceed 0.01. The CPU doesn't get touched, nor does the RAM. That said, I'd not reduce the performance of our disk in this system. I'd happily get a system with slightly less CPU and RAM though.