Those are good questions. I am not a scholar but still I will try to answer according to my understandings and my faith.

Your wife has a lot of rights on you. You have to treat her well and you have to satisfy her and you have to try your best to keep her happy. (For full details you can refer to rights of wife in Islam, you will find billions of articles on google.)

So having said that ... you have to keep your wife happy to be a good Muslim. Now this is where your decision making comes (Allah has given you the power to decide, and you will be questioned about your decisions).
If you decide to annoy your wife and marry another one, in my opinion, you are committing a sin, but again it depends upon so many different things, unfortunately life is very complex. e.g. May be by marrying another women, you are saving her dignity, providing her shelter, giving her respect which no one else was giving to her. Now if there is really a situation like this, may be annoying your wife is OK if she is unable to understand. But again, it is your call and you will be answerable.

One should personally discuss the situation with his wife, and if his wife is ok with it, he is lucky :)

But in my opinion if you are happily married, you should not think of polygamy as it is discouraged in Islam anyways. However, sometimes life puts you through a test, in that case one should pray that Allah gives him/her strength, wisdom and knowledge to make the right decision.

Adeel Ali