Sorry, 15k drives are optimized for server use. That means high iops in a heavily queued environment. In a desktop environment They do no better than a good conventional drive. The seek times are better because of the 15k rotation, but the data transfer times are not proportionally higher. I tried a 15k drive some time ago, and was disappointed.
Raid-0 will not help either. Raid-0 improves sequential throughput with large sequential files. That is not much help for the OS which primarily does lots of small reads and writes.
For the OS, nothing beats a good SSD. Take the money you would spend on a hardware raid card and spend it on a good SSD.
For photoshop , get as much ram as you can. The I/O you don't do by using ram is vastly faster.