Well, you will have to use RAID0.
You will have to start out by attaching your drives into your RAID controllers on your board. These should be labeled as IDE3 and IDE4. Or if you are using a RAID card, plug the drives into these controllers. You will not be using your normal IDE1 & IDE2 controllers.
After you have these in and ready to go, you have to boot into your BIOS and setup your RAID from here. After this, reboot and install XP. When you are installing XP, hit F6 to load your drivers for your RAID configuration.
Keep in mind that with a RAID0, there is only data striping, no redundency. So, if you lose one drive due to corruption, a virus, etc, then you will lose everything.
Since you have 2 drives, you can also use RAID1 disk mirroring, but you didn't mention that you wanted to try that. If you do, post again and I'll help you out if you need it.