1. NO mechanical HDD (that is, disks inside rotating on a spindle with motor, and heads moving over the disks) can possibly access its data as fast as the SATA II (more properly, SATA 3.0 Gb/s) interface. So there will be NO performance difference between SATA 3 Gb/s and SATA 6 Gb/s for devices of that type. SATA 6 Gb/s DOES make a lot of sense for SSD's, but there are good reasons NOT to put them into any RAID array, especially if you are just looking for faster performance.
2. The only RAID form that offers some speed advantage over non-RAID is RAID0. The actual speed-up may not be very large since current HDD's are really quite fast, but check the actual measured performance reported in reviews. Do NOT rely on manufacturer's specifications. Further, look closely at the review tests results, because the performance impact varies according to the types of files you are using, so you need to know what your dominant use will be before deciding how much improvement you might expect. Then decide whether the likely performance improvement of RAID0 is large enough to make it worth your while.
3. RAID0 is the MOST risky form of RAID. Typically it uses 2 disks in an array and BOTH must function correctly at all times. If EITHER one of them has bad data in a sector, at very minimum it means that whole file is lost. More importantly, a sector error CAN cause the ENTIRE data array to be lost (and this has happened to many). SO, if you go the RAID0 route, get real religious about maintaining a good current backup system so you can recover from complete data loss!
4. RAID1 is a mirroring system which allows you to recover from a disk error (bad sector) very quickly with virtually no downtime, so it is useful in places that cannot tolerate downtime. However, it is NOT a good substitute for a proper backup system. Those who use it in place of backups find out how wrong that is. RAID1 takes twice as much HDD to hold data (because it keeps 2 copies of everything) so it costs more.
5. RAID5 (and its more advanced cousin, RAID6) are quite fault-tolerant, with good ways to rebuild the complete dataset after ONE disk fails (RAID6 can even survive two disk failures). However, RAID5 requires a lot more computing power and tends to slow you down, not speed up. IF you have to rebuild the array after a failure, it can be done while still running, but it can take a very long time during which your "normal" operations are slowed down noticeably. RAID 5 or 6 is widely used in larger server systems where reliability and recovery for errors are vital. And those places typically have pro's running them that DO maintain a good backup system. Because these systems use several drive units, they get expensive to set up.
6. If you plan to use RAID abilities built into the mobo, read the specs carefully. Typically you can create RAID arrays using all of the ports on ONE of the mobo's controller chips, but not using ALL of the ports. So, for example, you might find you can RAID up to 4 HDD's on one SATA controller and its ports, but the extra SATA 6 Gb/s ports provided by a second controller chip cannot be used in any RAID set. Or, maybe they can be used only together, but not in combination with the ones from the first chip. This can have an impact on planning. For example, although a RAID5 system can be built with 3 or more HDD's, the classic form is 5 HDD's, However, many on-board RAID5 systems only work on a controller with 4 ports, so the 5-disk array version is impossible.
Hi, there is pretty much no way currently HDD come close to using SATAII, in fact the WD36gd (36gb raptor fisrt edition) will smoke most modern conventional (7200 640gb+) storage drives in terms of throughput; and that has PATA interface with a SATAI bridge.
I think that 6gbs SATA only be used effectively when multiplexing many drives of one SATA interface.
I would however say it is good get a motherboard which supports it, because you don't know what SSD technology is going to be used 5 years time.