I second nijineko. Egyptian magic revolved around "Hekau", which were apparently "words of power". A "syntactic magic" system would seem more setting-appropriate.
There's already an indie game called Sorcerer, about summoning demons, and how far a person would go for power. It's a pretty dark game, from what I hear. I don't know whether you wanted to go there, or whether Goetic entities in your world are more like djinn or pagan gods ... but "demons" connotes evil and corrupting influences.
Finally, I'm working on my own ritual magic system based around shamans conversing with lesser spirits, and calling upon greater spirits. I'm a little afraid of "netrunner syndrome", where the shaman does his thing and the other players take a soda break. Whether your entities count as NPCs or simply themes for certain powers (a la "vestiges" in D&D's Tome of Magic) will matter to the flavor of magic, and what players will put up with.
"On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."
- Charles Babbage (1791 - 1871)