RAID is a Redundant Array of Independent (or, sometimes, 'Inexpensive') Disks. It is a family of storage configurations that use multiple physical disk or disk-like devices, (HDD, SSD, SSHD, ect.) in different arrangements to create some certain desired storage solution. The most common are RAID 0 and RAID 1, with the former splitting (striping) data between two disks and the later writing two disks with identical data (mirroring). There are other types of RAID, but they are less common in desktop machines. RAID 0 is the option that offers a performance benefit, sometimes offering nearly double the performance in read and write, at least in theory. RAID 0 also has zero fault tolerance. If one of the two disks fails, all data is lost. RAID 1 has no performance benefit, but is redundant. With each of the two disks having a complete copy of your data, one can fail and you will be fine.
In theory, you can create RAID arrays from next to anything. More practically speaking, you are going to want same or similar disks/SSDs/whatever. An array is only as good as its weakest link, so pairing an 840 Pro with some SATA II OCZ SSD would be something of a waste, even if it did have a performance benefit from the pairing. I will say that it has been my experience that HDD are more forgiving to RAID than SSDs. Also, SSDs can't run TRIM commands in RAID so they pretty much go to crap if you let them go long enough, with write times becoming abysmal.
I assume your SSD is your boot drive, but are you also running PS from it? If you are running anything from the HDDs, you could create some type of RAID array there or by adding another disk and see some benefit. You could also create a RAID array by adding another SSD. I personally use 2 Samsung 840s in RAID 0 for my boot. It's incredibly fast, but stability is crap. I reinstall Windows about every two months.
If you are running an Intel chipset 68 or newer, you can also use Smart Response (often called Rapid Storage now) to 'accelerate' a traditional HDD or RAID array with a small SSD or SSD partition. All it is is a well done caching set up, and it is pretty useful at times.
My complete storage configuration is boot of 2x 120GB SSDs in RAID 0, mass storage of 2x 1TB HDD in RAID 0 with a 32GB SSD cache, and a SSHD used for an Ubuntu boot. I also have three external swap-bays, plus the optical drive, eSATA, ect, ect everyone has.
I would finish doing diagnostics before putting down cash on another drive or something.