if you're spending that much money on a premium laptop, you really should forgo the larger sized HDs for the faster spindle... If you need more space you can just get a usb mega sized HD. The hard drive is the slowest memory of the computer. Why skimp the speed on that??? (especially in raid you're going to theoretically see twice the difference in speed between the 5400 and the 7200 RPM).
Just a B.S.C.S.'s two cents...
Besides, you should really keep your OS on one drive and your data (photos, music, movies, etc.) on another. Think about a crash (virus/mechanical)... I've helped three of my buddies retreive data from dead drives this year alone...