From what I have understood there is a big difference between the Html.RenderPartial included in the ASP.NET MVC release and the HTML.RenderAction
Working with MVC requires much attention to not to shoot yourself in the foot. I mean it by the efficiency of the MVC products. In complex projects I would prefer to use RenderPartial rather than RenderAction. I use RenderPartial in which I use jQuery.ajax request (with Html.Action). It definitely works more efficiently than RenderAction. By this way you can put your Views into cache and then call jQuery.ajax. Try it yourselves. Ayende explains it clearly in Hibernating Rhinos.