Ram is quite different from the hard drive space you are talking about. Adding ram should not stress the system at all, but it won’t increase storage space either. It helps with moving information from the drives to the main processor, and more memory will speed up this process.
Make certain you get the correct ram for this system. I suggest going to crucial.com and entering the model HP you have into their memory guide. This is what I found using the model you gave us in your first post: http://www.crucial.com/store/listparts.aspx?model=Pavilion%207955
This is the page to start the search for their memory: http://www.crucial.com/
By the way, this tells you the appropriate type of memory. You don’t have to buy this exact chip from Crucial. Once you know the type, you can shop anywhere on line to find a good price. Newegg.com and zipzoomfly.com are popular spots for computer components.