If you are trying to add some speed to your computer, add a solid state hard drive. Use a free program such as acronis or partition magic to clone your existing hard to your new solid state hard drive.
My advise is to increase the size of your boot drive as not all data and programs can be installed to a second data drive. If you are building a new computer and using the ssd drive as the boot drive and a sata drive for the data drive, go with at least a 500gb ssd drive. I am seeing way too many customers filling up their 128gb ssd and 250gb ssd drives.
Remember to always back up your entire system, because a ssd drive is made up of memory locations. If the ssd drive fails, you will most likely lose all your data and nobody needs that terrible situation. I would use at least a 2tb external hard drive and the backup program in windows 7 or windows 10. If you don't want the responsibility of the backup drive, then go with something such as carbonite where your system backs up to the cloud. Carbonite generally has three choices and the middle one at approximately $99 a year makes the system image and backs up all data.
If you are not concerned with speed in a system, go with a 2tb or larger sata hard drive. It should be noted that the sata hard drives do a good job and will be less expensive then the larger ssd drives. No matter what you do, don't risk losing your data. I have seen it happen way too often.