Ok so I've got a 80gb (OS) and a 500gb (storage). I've got about 150gb free on the 500gb so I was thinking of getting another and putting them in RAID 0. But my question is, what would happen to all the data already on my HDD if I put it in RAID? and would it just show up as one 1tb drive?
If you initialized a new RAID level 0 volume on both drives your data would get nuked as the new volume was formatted. So, do backups!
Please note that RAID level 0 is striping, which alternates between drives for the drive that data is sent to. Any single disk failure in a RAID 0 configuration results in total data failure because no single drive has all of the data.
Well, if your using intel onboard raid you may be able to migrate over. You would have had to install the hard drives under ahci or raid mode in your bios. Add the second drive and use Intel Matrix storage console to migrate from JBOD to raid.