It's worth adding that there are two versions of Curse of the Dark Pharaoh. The original release was an excellent addition, as it added the highly thematic exhibit card class. However it suffers from dilution in an all-in game, as the chances to draw encounter which refer to the Exhibit items severely diminish.
The revised version of Curse of the Dark Pharaoh adds a special encounter to the streets and will not suffer from dilution. It's an excellent purchase.
More significantly, though, you should not overlook the possibility of using fan created content in your Arkham Horror game. Whilst there are plenty of articles of questionable quality, there are also a good number of custom expansions which add a great deal to the game and are of quality at least equal to anything that FFG has produced. This link (http://boardgamegeek.com/thread/898104/fans-of-fan-created-content-fofcc-post-here-ah-ver)This link contains links to the best of them, but for my money you should try The Door To Saturn, Lovecraft Country Horror or the Arkham Nightmare as your first experience of fan created content.