RAID systems were originally designed for large computers with many hard disks so that a variety of HD models could be used and, if one of these failed, it could be replaced without any data being lost. Essentially storage capacity was sacrificed to give increased reliability and versatility.
With two HDs on a PC, you are limited to RAID0 and RAID1 configurations (sometimes known respectively as 'striped' and 'mirrored'). The first is best avoided; there is no redundancy and a HD failure or error leads to complete data loss (unless it has all been backed up elsewhere, of course). RAID1 automatically keeps identical copies of everything on the two disks, so if one fails it can be replaced and the array rebuilt. It effectively makes them identical by trimming off the surplus capacity from the larger ie your array would store 160Gb. So it's better - but not essential - if the disks are the same size.
The alternative of course is to use the second disk to store compressed copies or images of the working drive. This is more flexible and doesn't lose any capacity, but requires more effort from the user. Your choice!