No extra performance in sata Raid 0 on Shuttle ST61G4

Just installed Hitachi DeskStar T7K500 320 GB as raid 0 striped in my
Shuttle ST61G4.
Installed the onboard Raid controller software fine.
All seems fine. I can see the C-drive as 600 Gb.
But but but In Sisoft Sandra 07 I only get 49 mb/sec and in HD Tach I
get burst speed = 50 mb/sec.
Shouldn't I at least be able to get 150 mb/sec.
What am I missing to setup?
Thanks!