Well, that being the case, I'll probably pickup an extra larger drive and use those in the RAID 1.5) Copy all the stuff out of the third drive over onto the RAID 1.
Software RAID 1 is an option for you. You want a set of identical drives, because the RAID 1 will be the size of the smallest drive. I'm not sure if you can partition a RAID 1 (I know you can't partition a RAID 0).
So you would need a pair of drives for Mac OS X, and a third drive for Windows.
Here's how to set it up:
1) Install the drives physically.
2) Insert your OS X install disk.
3) Copy everything on your main drive to the third drive.
4) Select the paired drives in Disk Utility (they should now be empty) and create a RAID 1.
5) Copy all the stuff out of the third drive over onto the RAID 1.
6) Boot into OS X.
7) Open the Boot Camp Assistant.
8) Install XP on the third drive.