No. But it is usually better to match drives then to mismatch.
If you mixed say a 60GB and a 240GB drive you wouldn't get 300GB of storage. You'd only get 120GB. Plus speeds are limited to the rate of the slowest drive in the array.
It is not 100% necessary, but it is recommended to have a matching pair. This helps reduce inconsistencies between the drive speeds, and will increase reliability.
Not necessary but recommended. I'm using 2 60GB vertex 3's. I'm happy with it so far.