Before my recent reversion to the Catholic Church, I spent several years in fellowship with many faithful God-loving brothers and sisters in Christ in two Evangelical Presbyterian churches. As I discuss my faith walk with various friends, I've found that Catholic and Reformed theology are each frequently misunderstood by adherents of the other.
Here's a link to a very readable essay by Jimmy Akin which discusses the foundation of Reformed Calvinist principles in light of what Catholics believe. Two things are especially worth noting:
** Each set of beliefs is sometimes discussed using a shorthand terminology (e.g. "Total Depravity", Mary as "Mediatrix") that can foster serious misunderstanding unless the fullness of the concept is understood as it is intended. This presents opportunities for charity, assumptions of good will, and willingness to learn on everyone's part.
** I was startled at how much actual overlap there is between the underpinnings of Catholic and Reformed beliefs. Check it out and see if you agree.
As an extra added bonus, here's Mark Shea on Is Sacramental Grace Magic?