Hardcoreware have taken a look at the Hitachi T7K250 SATA 300 drive, coupled with the NF4 chipset (which can handle the 3Gb/s transfer rates of SATA 300) in RAID. Sustained transfer rates for hard drives are still nowhere near 300MiB/s, but RAID some drives up and you can get some decent burst rates.
Well if you were expecting to see drive speeds double from using a 3 Gb/s transfer rate offered by SATA-II, you're going to be disappointed by these results. Although burst speed was significantly increased (especially in RAID 0), real world tests didn't show such a pronounced speed improvement.

However, there were definitely performance increases overall, in desktop and server tests. It's not exactly something to go head-over-heels about, but it's definitely there.
Get a couple more drives in the RAID array and SATA 300 will prove more useful.

[The review]