Looking for hardware suggestions for 5 terrabytes of storage.

another vote for RAID 4 configuration, coz RAID 4 improves performance by striping data across many disks in blocks, and provides fault tolerance through a dedicated parity disk. This makes it in some ways the "middle sibling" in a family of close relatives, RAID levels 3, 4 and 5. It is like RAID 3 except that it uses blocks instead of bytes for striping, and like RAID 5 except that it uses dedicated parity instead of distributed parity. Going from byte to block striping improves random access performance compared to RAID 3, but the dedicated parity disk remains a bottleneck, especially for random write performance. Fault tolerance, format efficiency and many other attributes are the same as for RAID 3 and RAID 5.

5TB isn't very much these days with 750GB disks available but it's more the throughput than the size that matters. You could for example get a ML350 3TB SATA storage server which is about £4500 but you would only have 6 spindles so it would hardly be high performance. the 6TB model will probably be out soon once the 1TB disks are available but it will still only be 6 spindles.

You really need to tell us what throughput you require rather than how much space you need.

For an inexpensive solution you may want to check out FreeNAS...
http://www.freenas.org/

For a quick evaluation there is a FreeNAS virtual machine available...
http://www.vmware.com/vmtn/appliances/directory/168