expanding raid

"If the hard disks are not the same capacity, the smaller size will be the amount of disk space used on both drives."

http://docs.info.apple.com/article.html?artnum=106594

(took only 2 minutes searching on Apple.com/support to find this, BTW.)

At any rate, the short answer is, you don't want to do RAID 0 (Striping). Other than in select multiuser or streaming media settings, it has no benefit, and is much riskier.

RAID 1 (Mirroring) can be useful in certain circumstances, but scheduled clones with SuperDuper are better for almost everyone.