Ok, so Im going to keep my student yamaha and get a new headjoint. But before I start trying, I wanted to see if any of you know what the following words mean, and how does these features affect the blowing, thus the sound:
high or low resistance
undercut (more or less, or none)
wave
riser (silver, gold, platinum)

Also. About size, the diameter and the thickness of the headjoint are the only things I should take into account?

Thanks a lot.

YOU need to TRY many head joints before choosing one that suits YOU.

The design details of a head are very complicated and there is no way you want to get bogged down with that unless you are going to make head joints. It is how the finished product works (and the inflated price!!!) that is important.

"high or low resistance"
Determined by many factors, including how the far side of the lip plate slopes away.
IMHO high resistance means that the player has to put more effort to achieve the same result. Perhaps the main reason a player might choose this is that he played a responsive head and got scared by what it was capable of, and did not want to put effort into learning to control it.

"undercut (more or less, or none)"
This can apply to tone holes (mostly on wooden or plastic instruments) &/or the embouchure hole, which is not only a thing to blow against to create the sound, but also a tone hole, the flute behaving as a tube that is open at BOTH ends. Undercutting refers to sloping or rounding off the edge at the bottom of a hole. It can be done to make the tone of notes more even, alter intonation, and to make the the instrument more responsive, maybe in some areas of the range more than others. Usually it is only the left and right (speaking of the flute in playing position) of the bottom of the embouchure hole that is done. Likewise, rounding the TOP of the embouchure hole is called overcutting. To do this part of manufacture WELL by hand can be time consuming and require a great deal of expertise. On the other hand, modern machines could do it in such a way as to exactly copy a head that is known to be fantastic. It is easy to damage an instrument with inappropriate undercutting. Some excellent designs involve very little or this cutting. The exact shape and amount of this cutting is a very complicated area of design, interacting with many other factors of design.

"wave"
a design of embouchure plate that is higher on the left and right sides of the embouchure hole. Although some people love this, it tends to put the lips in a straight-jacket, and limit the changes a player can make to his embouchure for versatility of sound. It is usually better to keep to the standard configurations that MOST players have found desirable.

"riser"
The `chimney` piece of metal between the main tubing of the head, and the lip plate that you rest your lip on.

"(silver, gold, platinum)"
A LOT of clever marketing centres around using different metals here. Think about it... a manufacturer can use say $50 worth of exotic metal and charge ten times that for it. Most of this preoccupation seems profit driven.

Both acoustic science and excellent research indicate that it is the shape of the AIR, inside the instrument, that is important, not the metal with which the container is made. The sound is made by the vibrating AIR, not vibrating solid material, as is the case of many other instruments such as piano, violin, bell. A lot of marketing hype and auto-suggestion (operating in player`s mind) surrounds this issue.

"About size, the diameter and the thickness of the headjoint are the only things I should take into account?"

Thickness of the tubing wall? Once again, best to stick with what is standard unless you have a very good reason to deviate. The standard has evolved as what the majority of players have found best.

Diameter of the tube?
The diameter of different heads varies minutely. It is normal for the diameter to have to be adjusted with special tools by a technician to make a new head a good fit into the body. It would only be extreme adjustments that may affect the head`s acoustic performance in any significant way.

Good post, now I don`t have to answer all of that! What ever would we do with out you Ted?!

PS. I agree with Ted, don`t get hung up on all the specs of a headjoint. Try one and if it works for you, then who cares what the specs are.

Sorry, I meant Ed.

Thanks a lot for that answer. Very informative. And thank you for taking the time.