1280x720 is HD while 720x480 is SD. You wouldn't try to convert your SD video to HD before uploading.

That makes sense.

I'm in the process of uploading the whole clip in mpeg format, using the following tag while the video is uploading, as YouTube suggests:

yt:crop=16:9

According the the Help Forum, this will zoom in on the 16:9 image and fill Youtube's screen (not sure if that means there will be a bit of quality loss). If that works, I'll upload it again in .avi format using the same tag.

That seems to have worked, though it looks to me like there's still a bit of vertical "squeezing." Also, the audio was disabled due to WMG copyrights (which I should've anticipated). I'm not really happy with the mpeg quality either, but will try uploading an .avi file and use only the camcorder audio without the studio track laid underneath, maybe it will get past YouTube's copyright filter.

Here's the full clip, for anyone who cares to view it and likes silent movies:

YouTube - BugleBoy

Thanks to everyone here who chimed in to help.

I read that somewhere as well Ed, but since I'd already been doing this and never had any problems with it I just stuck with the math, and kept doing it that way.
What is the reason behind the multiples of 8 ( or 4 as I read ), and downside of not complying with it?

I'm guessing because a byte is 8-bits and so multiples of 8 properly fill up bytes.

Video is compressed not by individual pixels, but in chunks known as "macroblocks"; traditionally these have been 16x16 or 8x8, though other sizes aren't unheard of. If you compress to a resolution where one, or both axes are not multiples of the macroblock size along said axis, the image gets padded with garbage (the reason for the padding is beyond me, but I suspect it's because of what Ed said above). The end result being that part of your bitrate is now going toward encoding that garbage, where a video matching block multiple sizes will dedicate the full data rate to the content.

The conventional recommendation is to stick to multiples of 16, though it seems more modern codecs use 8 unit blocks, but either way it's not the end of the world these days. Still nice to try and follow the rule, but at higher bitrates, destined for local playback or broadband delivery, there's not an enormous quality hit from using 640x360 instead of 640x352, 1920x1072 instead of 1920x1080, or any other resolution that's not mod 16, so you may not see a benefit from making the effort.