The new release (12 already) of Cinema4D restructures the different editions and promises more advanced rendering and character animation (among others).
A comparison between the different editions can be found on http://www.maxon.net/products/general-information/general-information/product-comparison.html.
Notable improvements for architectural visualization include support for real-world units (important when working with CAD models and realistic light falloff), photometric data (IES lights), color temperature support, white balance correction for camera. For animation, improved Dynamics, Deformations and Character Animation seem to be the biggest features.
For Architects and Designers, the "Visualize" or "Studio" versions seem most appropriate.
That said, the "Prime" version (the cheapest one) seems OK to get started (with Ambient Occlusion supported, but not Global Illumination).
Not free, not Open Source, but cross-platform (Win+OSX) and 64-bit support (which is good for large scenes and rendering).
A rendering from one of our students in 2009 (with r11, IIRC) shows what you can achieve with a basic understanding and some perseverance.