Well. All style rules for the class a.externallink will be applied during the 'pseudoclass states' (a.externallink:link a.externallink:hover a.externallink:active and a.externallink:visited) unless style rules defined for those pseudoclasses overide rules defined for the class.
Putting the definition for CSS3 target properties in a pseudoclass might cause seemingly erratic behaviour; I don't know whether a:link and a:visited are mutually exclusive... if they are, and you put the definition in :link, then your links would only open in a new window the first time they are visited.
I don't know whether the :hover state is dropped when a link is clicked: If it is, and you put the link in :hover or :active; your links would only open in a new window until they are clicked... i.e. never =P