Can there be space before and after a dash?


Both of these sentences have acceptable spacing for the dash (without space is more common):

Our new computers—which are really fast—will be popular.

Our new computers — which are really fast — will be popular.

This spacing isn't okay:

Wrong: Our new computers —which are really fast —will be popular.

Nor is this:

Wrong: Our new computers— which are really fast— will be popular.