Aspect ratios

[random lurker]

Sorry if this sounds newbiesh, but how do I decide which aspect ratios to use? I use TMPG Enc to make mpg files for XVCD's of movies in AVI format. Some of them have resolution like 576 by 320 and some have 512 by 384 when I get them. I know that 1:1 ratio should be used when width divided by length gives 1.3333. And that 4:3 should be used when it gives you something around 1.5. I'm right, right?

When should you use the 16:9 ratio? When the division gives you 1.8? When should 2.11:1 display used? Also, these aspect ratios are what you put into the source aspect ratios in the settings dialogue, right? And if you want to watch the movies on a standard TV, fullscreen, you should put the video aspect ratio to 1:1 in the setting dialogue, right?

Thanks in advance if you read this far and answered my questions.

[random lurker]

Oh and if it makes any difference, the resolution I use for the output MPG is 720x480