I am given to understand the drives have to be the same. I have RAID and bought 2 drives the same for this purpose. I don't know if it is possible to have RAID on two drives with different capacities it may well be possible but if it is possible you would lose some of the capacity from one of the drives as they have to be the same. So if you connected the 120 gig and the 160 gig in a RAID configuration you would, supposing it did work, lose at least 40 gigs.
If you really do want to try it I would buy another drive exactly the same as one you already have and take the surplus drive out, stick it in an enclosure and use it as an external hard drive presumming you don't have one already.
From my manual RAID 0 striping, 2 drives must be the same, RAID 1 mirroring 2 drives, does say if you're using a new drive with an existing one it must be either the same size or larger but I would think you would lose the extra space. RAID 0+1 striping and mirroring, needs 4 drives, RAID 5 needs minimum 3 identical hard drives, RAID 10 needs a minimum of 4 hard drives.
If you did RAID 0 I would think, if it worked and I'm not sure if it would as my manual definately states that for this they must be the same capacity, that you would end up with 2 drives the size of your lowest capacity one. Raid 0 as far as I know does not allow you to use 3 drives, according to my manual. Actually read my manual Page 5.21 click here
Yes, I think that for most common RAID configurations the disk size has to be the same. This doesn't mean that you can't use disks with different capacities, just that they will be trimmed down to that of the smaller or smallest. This is obvious with RAID 1, as each disk has a mirror copy of the data. With RAID 0 types, the data stream is split into equally-sized chunks or 'stripes', and sent sequentially to the component disks. Once the smallest disk is full, the process has to stop as you either lose part of the data stream, or lose track of where the 'stripes' are being stored.