Considering gaming engines, Unity is quite accessible. With the free (Indie) version you can do a lot, but it lacks some of the Pro features for more photo-realistic looks.
E.g. free version: decent shaders (diffuse, specular, normals, support for Substances), basic realtime shadows (hard shadows for directional lights), good workflow (integrate FBX or Collada models, update models from CAD/BIM software and reload automatically), quite powerful, intelligent scripting (without scripting, not so much you can really finish), integrated Asset store, integrated BEAST light mapping w. AO and soft shadows
Add to that for the Pro version: bounced light and final gathering in Light mapping, area lights (for light maps), full screen effects (e.g. AO, bloom, tone mapping), shadows for all light sources, blending between realtime and baked shadows with dual light maps, …
Price is reasonable.
For me, the workflow integration is very valuable. I can load an ArchiCAD BIM model through Cinema4D and host it as FBX inside Unity. I can still update the ArchiCAD model, merge it in Cinema4D which keeps changes and update the FBX model in Unity. Using Revit/3ds max works similar. SketchUp Pro is also straightforward to integrate.
REALIS3D is a realtime architectural visualisation system based on Unity.
An out-of-the box solution such as BIMx for ArchiCAD works also very well. It's a one-click thing though. Its good for casual users, but allows no in-depth customisation.