leandrodafontoura :
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.