Well, this is one of those crazy fun things that I had to attempt. My old system drive consisted of two 80 GB IDE drives in a mirror configuration. Pretty standard, but they were getting sluggish and I happen to have a few 320GB SATA drives doing nothing.
This is not a tutorial. This is a step by step account of a process I took to replace my disks; This is to be taken only as a suggestion as your configuration and mileage may vary. There may also be a better way of doing this.
So, lo and behold, I plugged them in to my eSATA ports and started rolling over! For the sake of this exercise the original drives will be sda, sdb. The new drives will be sdc and sdd. The raid device is md0.