How many times does a keyword or keyphrase need to be present in the text to be picked out by the search engines? How many times does it need to be presented to be seen as dominant?

Do you mean 'How many times does a keyword or keyphrase need to be present in the text to appear in search results' ? If the keyword is not that competative, once is enough. You can put it more than once, but don't spam. You should optimize your page with your keyword by putting it in the -
1. Title
2. Keyword in meta tag
3. Description in meta tag
4. Headings (h1, h2 etc.)
5. Body text
6. Hyperlinks
7. Page file name

checksum while you do want your keywords on your page it will really come down to the quality and quantity of your backlinks to the page and the site that will be more meaningful. You should still optimize your page in the places where Ronok mentioned (though I don't think the meta keywords tag is important at all) without stuffing keywords.

But don't try to over analyze the keyword density of your pages. Linking factors are going to have a much greater impact.

Alright, thanks again.

I have read several times, don't feel like digging up sources so you'll have to take my word for it(or leave it), that the <h1> tags are where search enginges are putting a lot of weight now, which is going to be your big headers and site headers.