Notty_lad is correct about the palm not touching the neck!
You should avoid clasping the neck of the guitar. Firstly, if you hold the guitar neck that way, then you would notice that stretching the fingers becomes difficult. Secondly when you have to move quickly between chords (especially barre chords) you have to slide the entire palm over the back of the neck which involves more friction! (Yeah, I happen to be a science student!..:p) Also, holding barre chords becomes more cumbersome!
With regards to the thumb, you can slide over the neck occasionally. You might wanna slide it over the neck to fret (or at times mute) the 5th or the 6th string or maybe while bending notes to add force. But otherwise, the thumb generally rests in the middle of the back of the neck in a comfortable position.
Just a couple of things though:
1) At the end of the day, you should be comfortable with the way you hold the instrument!..
2) Gohar is not the guy to look up to if you wanna learn guitar!..:p
We're sailing in the same boat...its just that you're looking for a destination and i am not!