Rene Descartes' basic position can be summarized in the following manner:
i) I can only know (or be certain) as to whether or not some statement or proposition (p) is true if I already know (am certain of) that God exists and is not a deceiver.
ii) I can only know (or be certain) that (i) God exists and is not a deceiver if I already know (am certain of) that every proposition I clearly and distinctly perceive is true.
In other words, i) depends on ii) and ii) depends on i). Neither i) nor ii) can get off the ground without the prior existence of i) or ii). Or, in other words, i) can't get off the ground without ii) and ii) can't get off the ground without i). It seems to follow, then, that neither can get off the ground.
A proposition P is metaphysically certain if and only if there is no other proposition R that is a reason for doubting P.
The Cartesian move is that, yes, we begin with only psychological certainty as to various clear and distinct propositions. The first thing we do with these psychologically certain propositions is prove that God exists. Then, with God's help, as it were, they become metaphysically certain as well. The question here, however, is that even if we accept psychological certainty, how is it, exactly, that such certainty “proves” God's existence? After all, psychological certainty alone can't prove God's existence or anything else for that matter. Only valid and true arguments can do that.
Then the Cartesian seems to move in another circle within the general Cartesian Circle. By proving God's existence from psychologically certain and therefore true propositions, we're giving a reason, apparently, for doubting all possible reasons for doubting those very propositions which prove God's existence. The propositions which took us to God are then blessed by God and therefore become certain. Thus:
God + psychological certainty = metaphysical certainty.
This could be seen as a virtuous, rather than a vicious, circle. Certain propositions lead us to God and God leads us the those very same propositions. And that, surely, is a classic case of the Cartesian Circle. I simply can't see how anyone has broken out of this very vicious Cartesian Circle. All that's has happened is we've expressed and codified it, not solved it.
Perhaps it all hinges on how psychologically certain propositions (if that's what they truly are) somehow “prove” God's existence. Clearly, simple psychological certainty of one or a hundred propositions can't prove anything, as I said. It all depends on what these propositions are. It also depends on whether or not these propositions assume the existence of God as well as the ability of God's existence to somehow give metaphysical legitimacy to the prior psychologically-certain propositions.