1. Do I really need Intel i7 3770k? or Intel i5 3750k is enough?
2. Is EVGA a good brand?
3. Do I really need a new soundcard? or an onboard sound card is enough?
4. Do I need to change stock fans on Cooler Master Haf X?
5. Is 850watts enough or do i need to increase it?
Additional Question 6. Which mobo is better? Asus P8z77-v-deluxe? Gigabyte Z77X UP5TH? Gigabyte Z77X UD5H? or Asrock Z77 Extreme 6?
1) An i5 is very powerful in its own right, and should serve you well for light video editing.
2) Anecdotally, I've used Evga graphics cards and had a good experience. I'd check reviews on Newegg to get a good idea.
3) Onboard sound is very capable these days and it's what most people use. Discrete soundcards are mostly for enthusiasts.
4) the HAF X is a very porous case and as a result has very good airflow (but you can also hear the fans more easily). The default fans should be just fine for cooling purposes.
5) 8 is likely more than you need with two graphics cards.