*sigh* I tried to stay from this conversation.. I really did..
The drives really don't have to be the same size. Most array controllers can handle a drive if it's a bigger capacity, the extra space will just go wasted and can't be used.
RAID1 is just a mirror to provide redundancy - it won't add capacity.. it is also (normally) limited to 2 drives in the array.
RAID0 can stripe the drives to add capacity and a slight performance boost but provides no data redundancy.. it also has the problem that if 1 drive fails in a RAID0 array, you lose the data on both drives, or if you have a 3 drive RAID0 array and one drive fails, you lose the data on all 3 drives. Basically, the more disks you add to a RAID0 array, the percentage chance of total data loss increases.
RAID5, if your controller supports it, can do more than 2 drives in an array (in fact, you need a minimum of 3 drives to set up initially) and proves n-1 expansion where n is the number of drives in the array.. example: you have 3 drives that are 500GB each.. your total space in the array is the same as if you had 3-1 (n-1 or 2) drives.. or 1TB for 3x500GB drives in a RAID5 array. RAID 5 provides for expansion and redundancy for single drive failure and is fast for reading data but a bit slower on writing data.
RAID10 or RAID01 requires a minimum of 4 drives and are slightly different in how they setup the array. They both do a combination of mirroring and striping of data but RAID10 is FAR superior in many ways than RAID01. I've written a long post about the differences before if you want to search the forums for it (too lazy to find it right now and don't want to type it all out again).
edit: as far as keeping existing data on the drive, a RAID0/Stripe set will require both drives to be clean - meaning data will be lost. A RAID1/mirror should be non-destructive on creation.. it shouldn't have to delete the data, but I've seen some silly implementations of RAID with onboard controllers in particular that for some reason, wipe both drives no matter which RAID type you select. Really though I understand why they do that. SATA drives tend to have a higher incidence of drive errors and failures and it's nice to start a RAID array with a clean slate.
You have a couple options depending on how your board is setup. If you have multiple SATA controllers, you can move your drive with the data to the independent/non-RAID controller ports and buy two new drives and connect them to the RAID controller ports and set them up in the array of your choosing (generally, I always recommend against RAID0 but people always seem to do it anyway).
You can find out if the RAID1 creation will be a destructive process or not and try to setup a RAID1 array if it isn't destructive - but one of your goals was later expansion by adding more drives so a RAID1 probably won't work for you.
If you take my advice and stay away from RAID0, you're left with either RAID5 or RAID10/01 and you would need to find out if your RAID controller supports these but they do require a minimum of 3 (for RAID5) or 4 (for RAID10/01) drives to start.
