Taking an extra semester to raise GPA
In general, taking longer to graduate is not a problem. However, your plan of retaking courses rather than taking new courses is a bit flawed. If you take more advanced courses and do well, then your earlier performance on more basic classes is less of a concern. It might not bring up your GPA as much, but taking new upper-level courses (and doing research as you have suggested) will give you more to talk about in a SOP, strengthen relationships with faculty, and better prepare you to do research at a graduate level.