That can indeed be used as you guessed, probably most frequently. In the example you give, I’d translate as “now you’re talking” most precisely.

But in a very different tone it could also mean “you don’t have to go on and on about it, I get it, I’ll do it”.

I think it’s not only about agreement.

If someone says “Pas besoin de me le dire deux fois !”, I would expect an action to follow. For instance, leaving from an unpleasant place/situation (e.g. stop doing chores), or to a pleasant one (e.g. getting ice cream).

I don’t feel like “Now you’re talking” or “You can say that again” express the same idea, but I never encountered them.

“Now you’re talking !” sounds like “Là tu me parles !”.

Pas besoin de me le dire deux fois !

although I hear it more often from native speakers in a reported form ("il n’a pas eu besoin de le dire deux fois") than in direct form, usually reflects strong agreement with a request/call to action, or agreement followed by action.

– Peux-tu aller chercher le dessert dans le frigo ?

– Pas besoin de me le dire deux fois !

Dire here is equivalent to demander.

(ne pas) dire deux fois refers to the possibility that the interlocutor might repeat their request. If they don’t need to repeat it, it means that you understood/agree perfectly and will act accordingly.

A slight variant that IMO is more frequent :

– As-tu vu que la piscine est de nouveau ouverte ?

– Ca, il ne faut pas me le dire deux fois ! Je vais tout de suite prendre un abonnement.

In contrast,

You can say that again! = You said it!

Tu l’as dit !

Je ne te le fais pas dire !

reflects strong agreement with an assertion or confirmation of a statement – no action is implied.

Now you’re talking

Là, tu m’intéresses

Là, ça devient intéressant

expresses interest in a suggestion or comment, sometimes as opposed to a previous one deemed less worthy.

You may also consider “Ce n’est pas tombé dans l’oreille d’un sourd”, which means that you were pleased to ear that and will keep that in mind.

When you’ve been told once ! 🙂 Kidding, but you can use it if it is the case. More precisely, you will say “oh ça va, pas la peine de me le dire 2 fois ! J’ai hâte de faire la vaisselle”. You could then add “Mais j’ai pas 4 bras !”. Dit d’un ton ironique en tirant la langue.
I vote -1 for this answer.