Almost all new material on YouTube or Vimeo is HD. Personally, unless there was a very compelling reason to shoot and produce in SD I'd use HD and always use square pixels. It's very easy to make things smaller, hard to make them bigger. Even the least expensive computer monitor has 4X the pixels of SD TV.
There's actually several years work in the this project. There's no way to go back and modify all that as HD. As I said, most of the original live footage was shot SD. Is there some way to modify that footage aside from adding some kind of weird side margins to the SD footage????
If all of your video footage is the same SD aspect ratio, then I would suggest working in that same aspect ratio, but larger. If playback is going to be on the computer only, you can do whatever size you want!
The problem is, if you're uploading to YouTube or some other video sharing service, then you are going to be constrained to their size requirements and, if it's YouTube, they will likely pillar box it (weird side margins) anyway.
If you want to keep all of your stuff the same size, then 720x534 is a perfectly reasonable square pixel resolution to work at. It matches the NTSC D1 standard.
Thanks. Whewwww. Yes, I know about those stupid borders. Sometimes I just create my own and bring the whole thing up to size specifically for youtube.