Sounds like a fun project!

The problem with having 8 buttons is that you will be stuck to one "key".
So for example if you have the notes Do, Re, Mi, Fa, Sol, La, Si, Do you will be stuck playing in Cmajor (you could also play in Am and in a couple of different modes, but that's a different story...).
If you want to play the "Happy birthday" song, and you want to start with Do, it means you need to play it in the key of Fa Major.
This means you would need to use the notes:
Do, Re, Mi, Fa, Sol, La, Sib, Do. <--- Notice the Si is FLAT!
You can take a look at the frequencies here:
http://www.phy.mtu.edu/~suits/notefreqs.htmlI guess a nice Octave would be from C4 to C5, but you can try and see which one you like the best...
Have you decided on your circuit?
Maybe the easiest way for you to play the notes is with the tone() command.
You can see here how it works:
http://arduino.cc/en/Tutorial/toneAnd here you can see an example of a simple "piano" with 3 buttons. Maybe you can adapt it to your project!
http://arduino.cc/en/Tutorial/Tone3When you have something ready, post your code and we can help you more from there!
Good Luck!
=)