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=tom...
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.
Ok, the RAID drives will just store data and programs. The OS is on the SSD. So I need something that will improve performance AND make copies of data in case of drive failure. I've assumed I will need RAID 5, but am wondering if I should just get a separate RAID controller card (diffently under $200).
I have seen so many conflicting statements here recently that I'm going to drop out. One guy claims that RAID0 doubles your throughput, another that it only added 30%. One poster has demonstrated that RAID1 slows down his system to 30% or so of its single-drive throughput, but we all _know_ that RAID1 has no effect on write speeds and can improve read speeds as the reads are spread between the two drives.
It was once common knowledge that any RAID level that used parity calculations (anything but zero or one) would benefit enormously from a RAID card with XOR processing on-board, because the XOR processing would put a noticeable load on your CPU and cause lag. Now I read that modern CPUs can handle the load without a hiccup.
So I don't know.
Now, if you want my best guess, totally unfounded, go buy one of FireWire's devices, or otherwise copy some specific case that someone has reported on, including the model of the motherboard. If you still want my best guess, RAID 0+1 can be handled by your motherboard's chipset.
But one major warning: RAID levels above zero will protect you against a single-spindle failure, but they are not a substitute for backups. Just built a RAID 1 so that there are two copies of every byte? Find, but do backups, too. This is not by best guess, but the result of many people's sad stories about lost data.