I read in a guide that reducing the amount of ads in a webpage would increase the revenue for each click since google would give priority to higher paying advertisers, and therefore avoiding the low paying advertisers. An also I read that the ads that are placed in a higher place in the pages give more revenue per click than the ones in the lower places. Can this be true?

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David Diez Art Prints
David Diez Music
This is starting to smell

Yes it can be true but really depends on your niche. More ads isnt always better.

Its the ads that appear in your page code first that provide higher earning clicks.

So the first ad of the page shows the most paying advertisers? Is so?

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David Diez Art Prints
David Diez Music
This is starting to smell

The first ad placed in your HTML, yes.

So what kind of ad you think should I put in that first place? I use now the keyword links one.

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David Diez Art Prints
David Diez Music
This is starting to smell

Its difficult to say without seeing your site. Feel free to post your URL and I'll try and make some recommendations.

Personally I like using 160x600 located on the left and then a 468x90 either before or after the main content on page.

I have several URLs, but in one of them I am using the combination that you said. It is this one http://www.freewebs.com/daviddiezmusic

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David Diez Art Prints
David Diez Music
This is starting to smell

Yeah you've got the positions right so I'd stick with that and see how it works out.