I have just gone from P IV to G5.

It was quite an expensive move !

Sonar is probably massive overkill unless you want to mix your own drum-kit, bass, vocals etc and requires quite a lot of user knowledge. Unless this is what you need Id go with something a little lighter exactly like guitar tracks.

Check this link out:
http://www.guitartricks.com/lesson.php?input=8471&s_id=71&prev_id=8099