RAID 0 on Pn32-E SLI

Picked up a brand new 1.5tb SATA HDD from Seagate and updated my rig.
I have 3 HDDs now, 250gb, 500gb and 1.5tb.
The 250gb has my XP and I don't want to touch it. But, I tried to make a RAID 0 set with the other two hard disks and when I created the set in the BIOS it shows the capacity to be 931gb instead of 2tb(500gb + 1.5tb).... Don't know why. Even XP recognizes it as 931gb.
Any help will be appreciated, thanks.....