Yes you would need to reinstall.
You'd set the SATA drives to RAID in the BIOS (instead of configured as IDE or AHCI).
Then reboot and go into the RAID utility (see the manual for how - sometimes cntr-I but it should say after the main logo shows, just below where it lists the physical drives. In there create a new array of type Raid0 and add both drives to that array as member disks.
Put the windows disk in and reboot - when it asks at the start to hit F6 (XP only), do so and then use a floppy to load the raid drivers (use motherboard cd to create this floppy on another computer). The windows will install on a virtual disk that the raid controller knows is both disks combined (but windows thinks it's 1 disk only).