Ok, Raid 0 sometimes reffered to as striping, is for speed, and large size, and raid 1 is known as mirroring, and is for redundancy (backup).
Ok here's the deal.. in a stripe set (raid0) the setup will only be as big as your smallest drive, and will only be as fast as your slowest drive...if you put an 80g and a 20g together in raid0, then you will only have a 40g stripeset... The other 60g on the 80drive will not be seen,(20g+20g) now on the other hand, if you put 2 80g's together then you would end up with 160g (closer to 157g or even as low as 148g after format).
Seek times will be only as fast as your slowest drive.
The way I did my raid is I got my raid array setup, formated the array just like a single drive, and took my old drive and ghosted it to the new array. If you are only going to get 1 drive, then you will have to backup your system and do a clean install,and restore your backup.....I recomend getting a matching drive for whatever you already have, unless it is getting too old/slow...and if you do choose to get two drives, tell the salesman that you want matching drives, and don't count on the salesperson to know what you are talking about, so look at the drive model numbers carefully, and ask their policy on returning drives if they don't match up.
Good luck and I hope this helps you clear the mud a little...cheers and Happy new Year
