The Existence of God

"On the other hand, returning to examine the idea I had of a perfect being, I found that existence was contained in it in the same way in which the equality of its three angles to two right angles is contained in the idea of a triangle, or that the equidistance of all its parts from its center is contained in the idea of a sphere, or even more plainly still, and that, consequently, it is, at the very least, just as certain that God, who is this perfect being, is or exists, as any demonstration in geometry could be."  pg. 21

After realizing that he is imperfect, Descartes determines that there is a more perfect being "upon which [he] depended, and from which [he] had acquired all..." (19).  This is God, whose existence is prior to and responsible for all others.  I find Descartes' argument to be based on assumptions rather than certainties.  For such a weighty topic, I expected more conclusive reasonings.  Descartes emphasizes the superiority of understanding to coming to know via senses and imagination, yet he fails to support his claim that God is the most perfect being with sound proofs (in my opinion).  It is, to him, one of those things that "you just know," like the fact that the sum of the three angles of a triangle equals the sum of two right angles.  In Phil Inq last semester, I had a problem with Descartes regarding his circular and flawed "I think, therefore I am" reasoning.  Again, I think that his ability to know and articulate his methodology is limited.  Is his reasoning sufficient or is it incomplete?