One key up from D is E.  In Good Hearted Woman, if you are playing A - A7 - E - D as the chords, move the final chorus up to B7 (no B, it's evil), A, and E. 

You're psyching yourself out. 

If I get the chance, I'll record it and shout out the chords I'm using.  I'll have to e-mail it to you, or whoever else wants it, as I don't have a Youtube account.   Or pop onto one of the Skype jams and I'll play the song. 

- Zurf

Key of G Major:
I    ii    iii    IV    V    vi    vii
Gmaj    Amin    Bmin    Cmaj    Dmaj    Emin    F#dim

Key of D Major:
I    ii    iii     IV     V      vi      vii
Dmaj    Emin    F#minGmaj Amaj Bmin C#dim

Gmaj is the I chord of Gmaj and the IV chord of Dmaj. Emin is the VI chord of Gmaj and the II chord of Dmaj, and so on.

Dean, I think you would transpose to E, but I'm not any good at music theory. I think Pete had a good definition for Modulation. Modulation is somewhat relative, if I'm correct. It depends on the listener, but there are a few rules to follow.
Modulation is when music changes keys (sometimes a temporary change for a mood change, or a permanent change 'grand finale'). For example, Any Man of Mine, Shania Twaine (got that in my head so I grabbed it) has a key change (grand finale) about two thirds into the song. It goes from Ab to Bb. This is a fairly easy key change because these two keys are closely related in the Circle of Fifths. These keys have a difference of two accidentals (Ab has four flatts and Bb has two flatts. 4-2=2) Ab and Bb have two chords in common, Eb and Bb. These two chords (Eb and Bb) would make good pivit chords. A pivit chord (like Pete explained) is a 'connecting' chord. The last chord before the Modulation. Most modulations last a couple of seconds, but that depends on the speed of the song (and modulation is really for the listener, so it also depends of when the listener notices the change.) Hope that helps, and be sure to not take all I say as Gospel, cause I'm wrong a lot! There are a couple of experts on here though, so they'll probably post in a bit.

If you are in the key of C, then if you transpose up to keys, it would be E.  One key would be D.  If you transpose it down one key it would be B.  If you transpose it down two keys, it would A. 

Remember that if you are using the major scale, you have a step pattern of whole, whole, half, whole, whole, whole, half.  So if you are using the major scale and transpose up one key, it would be one step (capo two frets higher when using the same chord shapes).  If you transpose up two keys, it would be two steps (capo four frets higher when using the same chord shapes).  But if you transpose up three keys, it would require putting a capo FIVE frets higher when using the same chord shapes. 

I hope that did not serve to confuse you.

Modulation, as I understand it, has to do with vibration reproduction out of speakers.  Or, it has to do with the width a string vibrates.  Neither of those meanings relates to the question I understood you to be asking, but it is possible (likely even) that someone, somewhere uses modulation to mean transpose. 

This may be a good topic to move to the Theory forum.

- Zurf