Your processor will only use as much RAM as it needs for the job. If it only needs 50% then that's all it's going to use. Buying more RAM won't do anything for you. The extra RAM will just sit there doing nothing while your processor continues to use only 3 or 4gb of what you've got. Wait until you start rendering much larger, complex scenes. If your RAM starts hitting the upper limit, which it won't, then you should start thinking about adding more RAM. But until then, you're fine. Adding more RAM will just be a waste of money.
As a point of reference, I have 12gb RAM in my machine but I have never used more than half, even on very large rendering jobs.