Putting the OS and your application programs on the SSD will make the system much faster to boot and to start up your programs. Ideally, you'd buy a drive that's large enough to hold all your programs, but if you can't afford that then buy one large enough to hold the programs you use most frequently.
RAID is not a backup - if your photos, music etc. are important to you then you need to back them up to an external drive. RAID protects against a drive failure, but it's worthless if you accidentally delete something, if your disk gets corrupted or virus-infected, if someone steals your system, if you have a power hit that damages the system, etc. etc.
Take that second hard drive and instead of using it as part of a RAID set, put it in an external case and use it for regular backups. If you can afford it, buy two external hard drives, alternate backups between them, and keep one of them offsite - that will protect your data from almost any imaginable threat.