Firstly these a "GREEN" drives and will have multiple issues raiding, they also run at 5900rpm, not the fastest drives to raid again, there are some posts out there of a newish Firmware for your drives (BETA) that could improve performance.....again maybe!
If you insist using these,... Have you looked at driver caching in the raid management utility (Intel Matrix/Rapid Storage) be aware this should only really be implemented if you have a UPS.
Can you get into the drives Bios to check how there set up regards , Noise/acoustic management, set all to high performance
Turn off all power saving features you can get your fingers near.
You could also try moving the non raid drive off the ICH10R controller (SATA Port) and put it on Marvell or GSATA port..... might get you a little more!
Raid 5 with three of these drive .... I could not even guess what speed you might get.... maybe 120, ....maybe 200, ... but with 3 better matched drives 250- 260 read /220 write is possible
Aussie Allan