You have a few options.
Set up the 2 Samsung HDD in Raid 0 array and put the WD in the Marvell Sata connection (navy blue) after installing Marvell Sata controller driver from the mobo CD and serves as a backup drive.
You can utilise the existing 3 hard drives to form a Raid 5 array which will give you performance as well as fault tolerance,make & model not a problem as long as they're the same capacity.
Even if you set up a Raid 5 or Raid 10 array ,you'll still need some form of backup (it can go wrong), so I'd suggest getting another Samsung to set up as a Raid 5 and use the WD as a back up drive.
Conclusion: option 1 should suffice.
Your CoolerMaster GX650W is Ok for now but only has 2 PCI express connections so if you want to have another GTX 560-Ti as well as some overclocking then you may need a beefier PSU with 4 PCI express connections such as this one http://www.ebuyer.com/product/234916