Como dizer "E como ! " em inglês

Sim, Sarah, você pode usar and how!

Mas parece que a expressão é um tanto regional. Veja o que diz um nova-iorquino no Wordreference:

I agree that it may be regional. I'm originally from New York City and I heard it a lot back there.

I've read that it was introduced into American English by German/Jewish immigrants, being a literal translation of the German phrase "und wie!" (used the same way to show emphatic agreement). Since NYC was the port of entry for many European immigrants, it seems like a plausible explanation.

Exemplos:

''Was Matt drunk?'' ''And how!'' [Longman]
“Are you looking forward to it?” “And how!” [Oxford]

Beyond belief

Exemplo: CUPCAKE": This townie isn't bothering you, right?
UHURA: Oh, beyond belief. But it's nothing I can't handle. Star Trek 2009

Sometimes "absolutely" can intensify the meaning of "yes":

If Sara becomes your girlfriend, you must be very happy.
B: Absolutely!

A corollary to answer by B:
“Absolutely yes, it would be my wet dream.”
E como! Seria meu sonho realizado!

Some otherr ways could express that in other contexts:
Precisely!
(meaning "I completely agree")

"No arguments here."
In the sense of ""I completely agree with you".

You said it.
(I completely agree with you just said)

Totally.

Couldn't have said It better!