DeadEye - Same issue as Sannin (I guess he/she didn't want a free rMP3).
That board is just an MP3 decoder + SD card socket. It doesn't do anything. You still need a controller.
So, $30 for that board, plus $30 for an official Arduino, and you're at $60. Then you will have to do the interface between the SD card and the MP3 decoder - tying up the Arduino which could be doing a lot of other things.
I'm definitely not putting down that product. It's great if you want to learn how to use SD cards and the MP3 decoder. Go nuts if that's what you want.
The rMP3 can run on it's own, or can be controlled by an Arduino (or another microcontroller) with very little interaction.
Bottom line:
You'll need more than just that $30 board to get MP3's playing. And you may not have much space/time left on your Arduino for anything else.
b