RAID 0 performanceWhile the block size can technically be as small as a byte, it is almost always a multiple of the hard disk sector size of 512 bytes. This lets each drive seek independently when randomly reading or writing data on the disk. How much the drives act independently depends on the access pattern from the file system level. For reads and writes that are larger than the stripe size, such as copying files or video playback, the disks will be seeking to the same position on each disk, so the seek time of the array will be the same as that of a single drive. For reads and writes that are smaller than the stripe size, such as database access, the drives will be able to seek independently. If the sectors accessed are spread evenly between the two drives, the apparent seek time of the array will be half that of a single drive (assuming the disks in the array have identical access time characteristics). The transfer speed of the array will be the transfer speed of all the disks added together, limited only by the speed of the RAID controller. Note that these performance scenarios are in the best case with optimal access patterns.
RAID 0 is useful for setups such as large read-only NFS server where mounting many disks is time-consuming or impossible and redundancy is irrelevant.
RAID 0 is also used in some gaming systems where performance is desired and data integrity is not very important. However, real-world tests with games have shown that RAID-0 performance gains are minimal, although some desktop applications will benefit. Another article examined these claims and concludes: "Striping does not always increase performance (in certain situations it will actually be slower than a non-RAID setup), but in most situations it will yield a significant improvement in performance." 
technically you don't need to have a RAID controller for "basic" RAID0 by making all of the disks in question dynamic disks and creating a striped volume across the entire array. of course, this is software controlled so you won't get the same "performance" increase as with a hardware controller. so you'd need a simple sata III card to add the 2 extra drives in.
if you're actually wanting to do hardware RAID then it would be suggested to have ALL of the drives on at least one RAID controller (if you can find one with 6 sata III ports, if not you surely can find a RAID controller with 3 and just buy 2 of them) otherwise you're not going to really be able to add the onboard drives into the array (i could be wrong here, i've never tried this, but based on what i do know i don't think you'd be able to have all 6 in the array if some are onboard and some are on a controller) and even if you could, i doubt you'd get the full performance out of the system