Philosophizing

So I had my first college class today. Philosophy 100. I'm already really enjoying it, the professor is fairly young and really sweet and very laid-back. The subject matter is challenging, but fascinating.
Right now we're reading about the ontological argument for the existence of God, which was put forth by St Anselm in the 11th century and later argued by various other philosophers, my favorite of which is Rowe. Anselm says that the existence of God is proven by his (or her) very definition. He defines God as "that than which no greater can be thought of", which is to say that God is a perfect being, that he is the most perfect thing possible. Anselm states that when you hear this phrase, "that than which no greater can be thought of", you understand it, and since you understand it it therefore exists in your mind. Since existence in reality is greater than existence in the mind alone, God by definition must exist. If he existed in the mind alone, then something greater (a thing existing in reality) would be possible. Since God is by definition the greatest possible thing, he must exist in reality. His definition proves his existence.
I don't know that I agree with this theorem. To begin with, I don't think that God is a perfect being, greater than anything else (if there even is a God, which I don't concede). But even if I accept Anselm's premise on faith alone, I can't accept that hearing the phrase "that than which no greater can be thought" automatically causes me to understand it. I understand the words, and the abstract idea, but that doesn't give me true understanding of God. God is supposedly perfect and infinite, and that puts him/her/it beyond human comprehension. It's like describing red to a person who has been blind all his life. He understands the word, and that apples and stoplights are red, and you can even tell him the color looks 'angry' or 'hungry' or 'bloody'. But this will do the blind man no good, he will still have no concept of red, because he has nothing to base his idea on. He has no knowledge of color. Thus do humans have no comprehension of a perfect being. We understand the concept, but not the truth. So how can God be said to exist in our minds?
But let's go one step further and agree that God is perfect and that he exists in our minds. The existence of a perfect God is a possible thing in reality, and he cannot, by his definition, be a non-existent thing. Anselm (as Rowe said) puts existence into the definition of God, he lists existence as a property of God by saying that existence is greater than non-existence and that god is the greatest thing. Therefore, his entire argument is flawed, because the premise that God is possible grants too much.
It's a tough argument because we can't say with any certainty that a perfect being is or is not possible in reality, and Anselm makes a good argument, but I just don't buy it. I have a harder time disagreeing with the Cosmological argument, but I'll go into that tomorrow.