The limiting factor may be texture size.  Depends on the render.  If you're doing a 12000 pixel close-up render of someone's face, and that face has a 4000 pixel texture, you may notice notice a loss of quality.

I notice it is mostly with clothing.  My renders are usually in the 5kx5k pixel range, and the faces are usually fine, but sometimes the skin on the neck, or the fabric of the collar and shoulders (if they show) isn't as high resolution as it needs to be.

IDL and complicated transmapped hair plus reflections and big textures has required over 10gb for a single character in a 5k x 5k render for me.

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PoserPro 2014, PS CS5.5 Ext, Nikon D300. Win 8, i7-4770 @ 3.4 GHz, AMD Radeon 8570, 12 GB RAM.