I want to try to my luck with JLPT N5 this December and I'm still not sure how much Kanji I supposed to memorize. I got the impression that for N5 I need to know around 100 Kanji, but I've already gone through half of Genki 1 and some of vocabulary list for N5 and most of those (especially verbs) using kanji for N4 and N3? Am I missing something or do I need to memorize those to for N5 too or those verbs and nouns that use more advanced Kanji will be written in Furigana at the exam?
Have you tried sample questions on the official site to get a sense of what the vocabulary is like? (Warning: the test applet plays very loud correct/incorrect sounds when you choose an answer!) From what I can tell, at N5 they give furigana for almost everything, except of course the kanji that you're specifically being asked to read. In the examples I looked at, I even saw 何 given furigana once - that's a grade 2, supposedly N5 kanji.
(Keep in mind that kanji are not written "in" furigana; rather, furigana are written above the kanji. Stuff like the page headers/footers etc. on the test might be written with advanced kanji, but you don't have to worry about that part - just read the questions.)
If you happen to recognize more kanji beyond what's explicitly expected, great! That can make it easier to understand the instructions, or (say) the text for the reading comprehension section. Keep in mind that kanji convey meaning as well as just pronunciation; when everything is in hiragana, it takes that much more effort to recognize words in the continuous stream of sound-symbols.