The "About This Computer" dialog tells you how much RAM your currently running applications are using. It has nothing to do with how much space on your hard drive your files (whether they be programs or documents like MP3s) are using. On Macs, if you click once on a file or folder, then either select "Get Info" from the File menu or use the keyboard shortcut [Apple/Command] + [i] you will get a smaller dialog that tells you how much space on your hard drive that object is occupying (among other useful information).

Your hard drive is a type of ROM (Read-Only Memory) as opposed to RAM (Random Access Memory). More RAM generally improves your computers performance by allowing more programs to be open simultaneously, etc., whereas ROM only determines how much stuff you can store.

RAM and ROM are both types of memory and therefore are measured in similar units (k or kb, MB, GB, etc) ... but otherwise two different beasts altogether. Hope this helps.