While a few renderers do have some gpu options, they're not always intended for production quality renders. You absolutely have to know the software you're using and fully understand the hardware. Nothing Apple ships is capable of more than draft renders at larger sizes even if you picked something that renders on the gpu. The exceptions would be maybe photoshop, after effects, nuke, etc. If we're talking about particle rendering and raytracing some objects being comped, the CUDA options can handle that. If we're talking about your typical offline renderer, that isn't gpu bound. That you asked means that you need to better understand your own workflow. By the time you load that up with textures that will hold up at A4 or A3 sizes, you will not be able to hold them in video ram anyway with cards that are available under OSX, even on a mac pro (maybe Quadro 4000 or some of the flashed cards). Mapped textures require a lot of memory. As blanka mentioned your speed of navigation in such a scene is more gpu dependent.