For casual video editing an i3 can do the job, its just going to take an unholy amount of time to encode. Some encoders will scale to as many cores as you can throw at them, some will only use two or four, so its important to know your software, if you have one that will take as many cores as you can throw at it you can expect close to linear scaling, double the cores half the time ish, you just need to decide if its worth the extra money to save a little extra time occasionally.
If you are looking to do lots of video work and that will be your primary use of the machine you should be looking into the software you plan to use and finding out what it recommends, some can get boosted by nVidia GPUs, some by AMD GPUs, but really for hard core video editing you should be looking at a multiprocessor system with a professional GPU in it.