Here's my translation (something idiomatic in Modern English while being as faithful to the original as possible):
Also that holy thing that is born of thee, will be called God's son.
To see why I translated it this way, let's start with a rough word by word translation, one for each half:
for þan | þt | halig | þe | of | þe | akenned | byð
forthe | that | holy thing | that | of | thee | akenned | be
byð | godes | sune | ge-nemned
be | god's | son | called
Now I'll refine the translation word by word:
for þan: The first two "words", "for þan", should be considered one word, "forthe(n)" because this other OE version of the text writes it as a single word "forðam". According to the OED, the word means "even", but I think Wiktionary gives a more apt translation: "also".
þt: Quite clearly "that". This book makes me think it was written as one character, "ꝥ", which only ever refers to "that".
halig: Only a noun really makes sense here. The relevant OED definition of holy (noun) is: "That which is holy; a holy thing." It's relevant to note that several Bibles use "holy thing" here as well.
þe: I also translate this as "that", because it makes sense. (If you have a better suggestion, leave a comment).
At this point, it makes sense to move some words...
byð: This is by+ð, or the verb be plus the archaic third person singular ending (written at various points in time as -ð, -þ, -th). According to Wiktionary "the present tense is used for the future, with context determining which tense is meant", and context says we should use the future tense in the translation: "will be".
of þe: I think this should be translated as "of thee". Nothing else makes much sense, and "of þe" is certainly translated as "of thee".
OK, second part now... I moved a word here too:
byð: As I said above, "will be" is the best translation.
ge-nemned: The verb here is "nemnen", which I translate as "called" because it matches definition 3 here. An explanation of the prefix ge- can be found here.
godes: This is a pretty clear cut possessive. While Old English didn't usually capitalize words mid-sentence, Modern English would definitely capitalize it: "God's".
sune: This is an old spelling of "son", used in OE and ME.