It is better if they are identical, but not essential. If they are not identical, your rAID array will be restricted to the performance of the lesser drive.
"So far, we've talked about several hard drives, but haven't gone into detail. You should principally set up all RAID modes with similar hard drives, as only then will you get maximum performance.
However, you can also combine different drives, with the smaller or slower drive being the determinant drive for the entire array. For example, one 30 GB and two 40 GB hard drives in RAID 0 will give you a total capacity of 90 GB, which is three times the capacity of the smallest hard drive.
The same applies to the combination of an old 40 GB hard drive with 5,400 rpm with a new model with 7,200 rpm. If you were to use two of the slower drives, the performance level would be the same. Replacing the older disk with a second and faster one would increase the performance."
From the link I gave above - you did notice the links to additional info at the bottom of the page?