Raid 0 - Erase all data?

As the message implies, only the new disks will be initialized.

Would it make a different to performance if the drives are not set as the boot drive?

Why would you boot from hard disks if you already have a much faster SSD? Setting up an array as bootable doen't make it faster or slower.

I wouldn't use the two harddrives as the boot drive - they would just be used as secondary drives as I don't have a lot of space on the ssd (only 60gb), which would remain the bootdrive. Would improve performance for the secondary drives if it was in raid?

Transfer rates will be faster if setup in RAID 0, but a single drive failure causes a complete loss of all data. Regardless of your setup, you need to backup important data to a removable device.

Its amazing how people dont answer a simple question.

Yes, setting the HDDs in RAID 0 will improve performance. It doesnt matter if they are your primary or secondary volume.

As said above, its really important to have a backup solution to a external hdd. I recomend getting only 1 more 1TB internal drive and getting also a external 1TB HDD. You can then, backup your current HDD in intirelly to the external hdd, create the RAID 0 and then transfer back your data to the RAID volume

Hmmm. I thought all the answers here, up to this point anyway, were straight forward and to the point. There is however a good argument that can be made about whether or not RAID 0 improves performance on a modern system with decent single drives for a normal user, or gamer.
But a good backup is a must for sure and moving the data to the array as you have described is the solution needed here if the OP decides to use RAID 0.

To the OP, you understand that you are splitting all your data in half across 2 drives in a RAID 0 situation, literally byte per byte, and if the array is broken somehow(this is what most usually happens), or a drive fails(unlikely, but it can happen) you will LOSE EVERYTHING THAT IS ON THE ARRAY with virtually no chance of ever recovering it.
Be warned and be prepared.

Thanks for all the advice guys! This has really helped me out.

Raid 0 will improve performance for the HDD's, but, unless your mobo doesnt have 2 sata controllers then your SSD will not be in AHCI mode and "lose" perfomance, not much, but its there.

Frankly, RAID0 is not a good choice for data storage place because if only one drive in it fails, you'll lost all data from the entire RAID. And modern drives fail too often.