Tengo affeción para ti

Would it be OK to say

  Tengo affeción por ti.

HI Izanoni1, the word "affeción" does not exist in Spanish. I suppose you mean afecto.

Tengo/siento afecto por ti.

This is posible (better with siento), would sound very formal though, and nothing sentimental involved.

Does Me siento atraído por ti translate to I feel attracted to you/I feel attraction for you?

Yes, that is correct

Me cae muy bien.

Me gusta(s) tú. (culpa mía)

Though Spanish can certainly express the concept of "fancy" in this context, I don't think that they have a verb that would be translated as "fancy" in various other contexts.

How about "me quedo prendado de tí"

I´m not sure if this phrase is awkward in Spanish, pues quizás:

          **Tengo affeción para tú**

or maybe using the verb form:

          **Me afectas**

or

          **Me has afectado**    

Like I said, I am not sure how well that translates, and any feedback from someone more experienced with the language would be appreciated.

Tengo affeción para ti

That would be "I have affection to give you". Not a good choice.

Me afectas

Me has afectado

Those two sound like someone has done something bad to you.

"I fancy you" would simply be:

Me gustas Me siento atraído por ti

i te apetece

I think "Te deseo" is closer than "Me gustas":

http://www.urbandictionary.com/define.php?term=fancy

Tengo affeción para ti

That would be "I have affection to give you". Not a good choice.

Would it be OK to say

      Tengo affeción por ti.

"I fancy you" would simply be:

Me gustas Me siento atraído por ti

Does Me siento atraído por ti translate to I feel attracted to you/I feel attraction for you?