Go to iTunes, pull down preferences and you will see an option to select iTunes as the default format for downloading from the internet. Click in the box and all new downloads will belong to iTunes. This will not, however, change the format for the files already downloaded. However, the format does not prevent you from using Add to Library in iTunes which will allow you to consolidate the playing of all MP3 files in the one place.

You will have to refresh the selection of iTunes as the default format every time you upgrade and the iTunes tag can sometimes slip off Quicktime files so you may find you are using Add To Library quite a lot.

Hope that helps.