I don't really know the right answer, but I think it depends on the tv you're planning on using. Not all of them are 4:3, for example this chart shows many screen dimensions and ratios- link

A few years ago I used to make slides that appeared on closed circuit tv. I don't remember the exact dimensions but photoshop has a lot of presets for tvs of various sorts. When creating a new file click the preset menu at the top of that window, select film & video, and then the size menu will be updated with a large number of options.

Thanks for the help :) I don't think I explained my question very well, so I'll try again.

I know 640x480 is 4:3, what I don't know is how that will look when displayed on a TV screen.

For example, if the TV is 4:3 but it's a 43cm screen, will the image be full screen on the TV or will it display at the exact resolution of 640x480, which would look like a tiny display area on a big TV?

The Photoshop presets are handy so thanks tanuki, but I still need to know what res to work at :)

I'll burn my first two scenes to DVD and have a look, and let you know what I discover (in case anyone else is interested). Thanks again!