Okay, I admit I use DAZ Studio not Poser so this question may seem stupid.  If the OP is rendering a 1280x720 pixel then what difference does the DPI make, its just a number stuck in the meta data of the final image.  Now to the OP's problem, as hborre has already mentioned texture sizes and surface setting tend to have a bigger impact on rendering than the number of objects (unless you have a few dozen high poly figures and props).  There is seldom any reason to use texture maps that have a larger resolution than the image you are trying to render.  Displacement maps and in some cases transparencies are the exception to this.