You do not want a RAID 3 setup. It will not capture HD video anywhere near as fast as you need.
RAID 3 is similar to RAID 5 in that multiple drives are used with parity information on the drives to provide redundancy. In RAID 3, all the parity information is stored on one drive. Since the parity information must be written along with all the data, the parity drive is a bottleneck, slowing transaction rates for the entire array to that of a single drive. In addition, to achieve maximum possible performance in the RAID 3 setup, the hard drives must be spindle-synced, which most cards no longer support.
RAID 5 has distributed parity, where the parity blocks are distributed over all the drives. In this setup, the theoretical write speed would be (n-1) * (write speed of 1 drive), where n is the number of drives. This would be further limited by the speed at which the parity data can be calculated.
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AMCC 3Ware 9650SE-8L is an 8-drive SATA-300 RAID card incorporating RAID 5 capability (as well as RAID 6). The interface on the card is PCIe x4, which can plug into a PCIe x4, x8, or x16 slot. (Be aware that some motherboards won't accept anything other than a video card in their PCIe x16 slot, so check motherboard compatiblity before you try that). The manufacturer claims this card can sustain 700MB/sec reads and 600MB/sec writes in RAID
6. Should be faster in RAID 5 provided the drives can keep up with it.
Uncompressed 1920 x 1080 HD (RGB) @ 30fps is 186MB/sec. Thus this card should handle this data stream no problem. (By the way, that's 672GB per hour of captured footage ... with 8x 750GB Seagate Barracudas that only gives you space to capture for a little less than 8 hours before you fill up the 5.25 TB).
As far as what you read about RAID 3 being higher performance than RAID 5, you have to remember that many of the RAID descriptions and write-ups on the Internet were written a long time ago, when controller computational power was tiny. At that time, anything physically setup in in the array that simplified the design and reduced the firmware's job increased speed. RAID 3, with a single parity drive, synced spindles, and XOR in dedicated silicon reduced the firmware to simple block transfer.
This is no longer the case today. Today's RAID processors and PCIe interfaces can out-calculate any previous generation of RAID card, making simplifications in the array design unnecessary to achieve high performance. No one even contemplated RAID 6 until just a few years ago because the computational cost was prohibitive. Today, many manufacturers are making RAID 6 controllers.