This is a hugely subjective question, and there is nor right or wrong answer and it very heavily depends on your system and usage patterns.
Assuming its a reasonably modern system (IE, sata) you may want to consider using a SSD of some sort (even a 40-80 gb drive) for the things you need running faster and using a larger drive for storing finished work. With pretty much anything short of the latest bleeding edge hardware (IE sandy bridge or bulldozer based systems), chances are you'll be maxing out your drive interface's bandwith with an SSD, and should minimize loading times, assuming your disk is your current bottleneck, and you have the files you need on the disk.
Large hard drives are relatively inexpensive, both internal and external - i use an internal drive for things i want stored (ie, in theory i can afford to lose) and an external drive (or a good unlimited cloud based backup service) for a second line of backup. Once again, this depends on your needs