If you intend to do any 3D modeling, high-resolution image manipulation, or digital video editing, get as much RAM as you can afford. To cut initial costs, make sure your system is easily expandable to allow more RAM to be added at a later date. Then you can purchase only what you need now, and add the rest later. The hard drive allows you to permanently store information that you would otherwise lose from memory when the power goes off. This storage space, sometimes called hard disk space, is completely different from memory.
People mix up the two quite often, but they're actually quite easy to differentiate.
If the file you are working on has not been saved to the hard disk, it will be gone forever when you turn your machine's power off. As has been said recently, just like closet space, you can never have enough hard-disk space. The point here is that you need more hard-disk space than you think you do. Most systems come standard today with a minimum of about 125 MB of storage space, with 250MB being more of the norm. You'll be happy that you did later on.