HTML email and cache problem

Hi to all,
i need to customize email sent to users when i register them on the website
Now i can do that going to admin/user/settings. But i need to customize it in order to allows drupal to send html mail. Is it possible that?

Another problem. I'm creating some modules, but sometimes I upload edit I must force users to perform a reload (F5) to take changes. How to avoid caching?

regards

vg