Directory rights

Many users of WP, like me, are not very PHP or UNIX savvy, and questions on directory and file access rights pop up now and then.

Could anyone from the development team summarize the correct settings for the WordPress directories and files to provide

1. Cache
2. Image upload
3. Backup
4. Best possible security

I think the best format would be the octal form < 777, as many of us have our blog at service provider ( web hotel ).

I know this may be dependent on the SP:s settings, but some general rules should apply.