affiliate advertisements

I was looking at some wordpress themes and there seem to be many of them that have places to put advertisements in the header area of the theme. Most have spots on the right side of the header FYI. What do you guys think of putting affiliate advertisements in the header area of a blog?

I was doing some research for my upcoming blog and I was looking at Wordpress plugins and saw a couple that were for rotating advertisements. Is it a good idea to have a couple different affiliate ads and have them rotated in being displayed around your blog?

I was wanting to ask for some help. I want to know if it is ok to put two affiliate advertisements on my blog for the same kind of things. Not the same advertiser but 2 different advertisers. Can I put two affiliate advertisements for web hosting on the same blog for an example?