Conditions of Excerpt and More tags

Pretty simple; I just want to set the excerpt to show only if I enter in an excerpt to show; I hate the fact that the excerpts that wordpress makes don’t include pictures or links.

What is the php to make that possible?

In english,

if there is an excerpt, show the excerpt. If there isnt’ an excerpt, show the content before the more tag. If there isn’t a more tag, just show the content as is.