Below 15% makes defrag difficult.
Although I'm a big RAID 0 fan I'm not sure I would raid 0 two 500GB drives with nothing left for redundancy. If you have 1TB of backup storage available somewhere, go for it. Otherwise put em as single drives and back one up to the other.
I'm also a big fan of having one large volume rather than partitioning up. There is a limit to the benefit though... not sure where, but 500GB is definately past it. I wouldn't Raid 0, or JBOD just to get both of those into one volume. No real benefits.
I would 2xsingle drive them myself unless I had 1TB of backup storage available then I would consider the raid 0.