I think you might be experiencing diffuse hair loss to the point where over time you will need a full cap. It looks like your sides/back are thinning equally with your top.

If your sides/back keep thinning, I would just say shave it all and go full cap.

You'll have a hell of a time trying to blend a topper with your sides/back if they get so thin they're hardly there.

Also with your hair that short (and thin), blending will probably only be harder.

You can see a few good full caps here:
http://hairjive.com/blog/2009/10/10/the-...ttachment/
http://hairjive.com/blog/2009/09/24/the-...cial-hair/
http://hairjive.com/blog/2009/11/07/full...oto-shoot/
http://hairjive.com/blog/2010/04/22/gues...-full-cap/