I agree with the above -- an external drive is the method of choice.
In addition, I'd suggest that if you have either a standing computer or a decent-size laptop, you should get a second "data" internal drive to store your immediate downloads (as well as your catalog folders). This way you can work on your current shoots on a fast internal system and then, once you have converted jpegs for whatever use you can move the folders to the external drive.
One other note -- you say you convert to jpegs, which is good for when you need a jpeg for a specific purpose, but a question is are you doing this to all your files, just to have a jpeg copy of everything? That to me is a waste of space/resources, because you don't really need a jpeg unless, well, you need one of a shot for something specifically, such as to upload to the Web or email to a friend, in which case a very small/compressed file is sufficient, or you need a large/high quality jpeg for printing externally (DPP can handle home printing). So, take a long-term look at what you are doing (your workflow) and figure out how to do your work efficiently. And, yeah, you have to spend your bucks to keep up with technology. But, if you choose wisely now you can set yourself up to be good for some time.