Professional graphics cards a "must" for rendering static environments?

Pro cards are usually slightly declocked versus gaming counterparts, and their drivers updated less often, since the drivers have to go through a more extensive verification process. The performance difference is fairly small but measurable, but since correct results matter more than a few seconds off of a one minute render, this is accepted.

The pro cards (and most importantly, their drivers) are guaranteed by the application software vendors to work with their application, and provide verifiable results (CAD), and/or with correct color matching (Publishing). Pro cards (and their supporting applications and drivers) provide the capability to calibrate colors, which is usually a functionality that is skipped or glossed over for the gaming cards.

While the gaming equivalents usually work fine, the exact answer to your question of need is more a matter of correctness than high performance. Whether a gaming card substitute works correctly is a question best answered by either the application vendor or the power users of that software (and its different add-ons and modules).

Things change fast, and I'm probably behind the times... I hope I am only slightly wrong with what I say.

For the most part, high end cards a great at rendering in the style of OpenGl or DirectX - scan line conversion, with vertex shaders and all that, and do so very well at high frame rates. High quality photo-real rendering usually involves ray tracing, photon mapping, Metropolis algorithms and other techniques that don't translate well into hardware, and rarely can be done real time (w/o spending big, big bucks). The high end card may pay for itself during the modeling and lighting phase, where the user interactively alters meshes, moves lights, applies textures etc. while the final output render would be done in mostly in software. Typically, a designer spends a lot of time modeling and texturing and adjusting the view, and (at least on a good day) taps the "render" button only once.

The essential stupendous point: be clear, when deciding on a graphics card, on understanding whether it's benefit is for during interactive scene design, or for the final renders.