RAID Size Change

I recently lost an HD and RAID controller(Promise 20378) on my Asus MB. It was setup as a RAID1. I need to replace some hardware, but how I do it is the question. If I get another 250gig HD and a Fasttrak controller I should be back where I left off. But I could use more space(transfering family vids to DVD). Here’s my question. If I connect 320gig HD and recreate the RAID1 and then replace the 250 with another 320 will the OS see a 250 or a 320?
Thank You
Mark