How much money do you have to spend?
How many drives are you expecting to use?
What RAID level are you expecting to run?
The answer can vary greatly depending on these parameters. For example, to "What interface is best," you should have an 8-lane PCI Express card (only because 16-lane ones aren't out yet) with SAS drives, but that will raise your cost to several thousand dollars between the card, drives, and enclosure. You could look at this member's proposed config: http://www.tomshardware.com/forum/forum2.php?config=tomshardwareus.inc&cat=32&post=271964&page=1&p=1&sondage=0&owntopic=1&trash=0&trash_post=0&print=0&numreponse=0"e_only=0&new=0&nojs=0
On the low end, FireWire2 has a great device that will RAID up to 5 SATA drives and present them as a single SATA device. Under $200.