A way to express an overreaction to something positive

I don't think any of the answers provided really express over-reaction to positive news. You could still use jubilant, ecstatic or bursting regardless of whether or not the subject is justifiably excited. I believe gushing is the adjective you are looking for:

Gushing

(Of speech or writing) effusive or exaggeratedly enthusiastic

Or the verb form:

Gush

To make an excessive display of sentiment or enthusiasm

And an example sentence:

"Bob was gushing over Radiohead's performance last night. I thought that the performance was good, but it wasn't really worthy of the amount of praise that he put forth."

Ecstatic comes to mind for a single word

Feeling or expressing overwhelming happiness or joyful excitement.

Examples

• When my eyes finally adjusted I was ecstatic with happiness.
• Here's how ecstatic Boston fans got the news from their morning paper.
• But I hoped he felt the same as I had, ecstatic and bubbling with happiness.

To burst into tears of joy may be a useful expression:

describes that you are unable to control your feelings because you are too happy and you cannot but show it.

Also: to be beside oneself with joy

beside oneself: almost out of one's senses from a strong emotion, as from joy, delight, anger, fear, or grief.

You can use overexcited to describe someone who is reacting excessively to a positive or negative situation.

For example:

"Calm down, stop getting overexcited!"

How about 'ebullient' (adj.); 'ebullience' (noun)--the quality of lively or enthusiastic expression of thoughts or feelings. e.g., 'he was ebullient over/at the news of his promotion to manager.'