to me, niche applies to a one-subject website or blog. i run an article directory with many different topics - do you think this would perform less than a dedicated single-niche blog as it has less frequently updated content? i have just started a newton faulkner fan site (amazing british guitarist/singer) and have put adsense on that. when i get traffic on my sites i'm planning on guaging which is performing the best and this might give me a better idea. incidentally, does anybody know the general CPC for music sites? i'm guessing it's low...
A real niche is some term that is popular and has good paying ads. Niche for affiliate program is just a way to focus on certain product to get the most traffic