Hard Drive Question

I will have 2 Raptors and they both spin at 10,000 RPM. Should one Raptor have a capacity of 74 GB or both at 150 GB? I will have a RAID setup on the Asus Maximus Extreme. Will it be necessary to have both 150 GB or just the 74 GB for the OS? I want storage also. Will the RAID set up reconise that I have 2 HDDs and distribute the OS and other files evenly on both disks?
Please Explain because I don't get it much.
Please Explain because I don't get it much.
