Scawny,
I think I understand your question, please let me know if I do not answer your question.
First which you probably already know:
1. External style sheets are for use on multiple pages which have the same style.
2. CSS that is on the page itself (known as embedded) will over-ride external style sheets.
So I would use externally linked style sheets on any pages that have the same type of style. Makes it much easier to manage if you want to make a change. You will only be making one change to an externally linked CSS file that will affect any pages that are linked to it.
So if you are using an externally linked CSS file add this code between the head tags and after your page title.
<link rel="stylesheet" type="text/css" href="myexternalcss.css" />
At this point I would remove all embedded CSS except for the styling that you want to over ride on the specific page. (remember to put your embedded CSS after your linked CSS in the "head" part of your document.
Then I would use internal (or embedded) CSS for any style that you would like to over ride in your external style sheet. If you go this route I would put your embedded CSS after your externally linked CSS to make sure that your over ride performs correctly (this is known as cascading). If you had several pages to over ride then I would make another external CSS sheet "override.css" and use specificity to over ride the CSS. I would also put the "override.css" external link after your regular external style sheet link.
reference:
This link to Smashing Magazine explaines CSS Specifity.
Link to Boogie Jack.com that explains CSS Cascading order.
Good luck and let me know if you have more questions.
-Tim