On your video, it really is amazing how much better a good mic makes the whole thing -- the video part is kind of irrelevant, but being able to clearly hear you speaking without a ground loop hum, wind noise (people doing videos outdoors for some reason!), fan noise from a computer and built-in mic, etc. is a big improvement over many online videos.
Even I was amazed by how much better the audio became. The camera still recorded audio for sync purposes, and it was remarkable how much more "present" the audio from the Zoom H4n was versus the camera audio. I highly recommend getting some sort of dedicated audio solution for anybody doing online video. Something like the Zoom H1 can be had for less than $100, and it will make a world of difference.
I really like the Blue Snowball ($50-80) USB microphones for a lot of purposes. Not as portable as an H1, but really convenient.
(I love my H4ns, though -- I use them for everything. For conferences I usually have cameras with shotguns on L, board audio on R, and a zoom doing the same thing near the board. I still use a Sony HDV with 3 big chips and uncompressed HDMI out to an Atomos Ninja, though, and HDV tapes for backup. Picking the best audio track and getting it all in sync is a pain, though.)
