Frege discusses the ideas (Vorstellungen) in Logical Investigations part I: Thoughts. Descartes discusses the ideas (the imagination) in Meditation VI. We have to find a similarity and a difference between these two with quotes from the texts. I simply can't find a difference between them.
For Frege, thought (Gedanke) is objective while idea (Vorstellung) is mental.
The thought is the "conceptual content" expressed by a sentence.
Ideas live in the so-called second realm of mental facts, while thoughts live in the third realm of timelessly truth.
According to Descartes’ Theory of Ideas, ideas are modes of the thinking substance, and thus they are in the mind.
Up to now, no differences. But the basic difference is that in Descartes' ontology there is no third world: concepts (and thoughts) are mental entities as well.
Compare with Locke's theory of ideas (quite similar to Descartes on this point) :
“words in their primary or immediate signification signify nothing but the ideas in the mind of him that uses them”.
This, in a nutshell, for Descartes ideas are the meaning of words/sentences, while for Frege they are not.
When Descartes talks about an idea not as an act of representing but as the very object of this act, he is not so far from Frege. By this I mean that Desacrtes does not totally reduce thought to " something that occurs in my mind".
According to Descartes, ideas have a double status:
(1) an idea first has formal reality, as a mental item, a mode of the soul ( thinking substance)
(2) it also has objective reality , as a representation of an object outside the mind, and, as Desacrtes says sometimes , as the object itself ( inasmuch as represented).
The subjective realty of my ideas depends on my mind. The objective reality ( and degree of perfection) does not.
Descartes uses this distinction to show that I cannot be the cause of my idea of God : this idea contains an infinite objective reality ( as a representation of an infinite being); as a finite being ( having only finite perfection) I cannot have caused such an effect ( in virtue of the " axiom" : the effect cannot be more perfect than the cause).