In my opinion, the HD part of reading an MP3 is not that significant - an average song would take 3-4 MB of space, and is read to the memory once every 3-4 minutes (Average length of song), so its not as bad as my preceders answered.
About the CPU usage, you should keep in mind that MP3 files were designed to work on 100 MHz computers (yes, those antique P1 processors), and should not take much more than any "regular" program.
Just follow your CPU usage monitor and see how much it takes to simply move your mouse around... With my E6300 processor it can take as much as 20% of CPU capacity if you move it around real fast... that's approximately 300 MHz (sounds farfetched, but I just checked it...)