|First of all, are you planning on installing WinXP on the same partition as Windows 7?|
Next, WinXP would require you to set up the RAID array at the beginning of the install. You also need to install SATA controller drivers when using SATA RAID.
As StuartS has already pointed out, the benefits of RAID0 are questionable.
If you insist on doing that anyway then I THINK you will need to partition both disks into equal primary partitions. Then install WinXP first and then Windows 7 after.
Big waste of time, if you ask me.
What else is going on the drives? Is this a game rig?
You might do just as well by playing with cluster sizes tailored to the games you need to load. Or get yourself a solid state drive. Link below.