Sounds like you need to read a good article on RAID, thatbox. When you use RAID, you are taking two drives and making them one as far as your OS is concerned. With RAID 0, your computer reads from and writes to each drive sequentially, which is what makes it so fast. Once one drive runs out of space, you no longer have the ability to RAID them. Since both of your drives are recognized as one, the extra space is pretty much a waste. That's why you should use drives that are the same size.
If I were you, I would see two choices:get another 100 gig and boot off the 30, or you could get another 30 and use the 100 for storage.