Floating-point depth buffers would enhance range if they actually stored non-normalized depth values. You have a choice between 32-bit fixed-point or 32-bit floating-point depth, for all other bit-depths the depth buffer is always fixed-point. So compared to a 24-bit or 16-bit depth buffer, a floating-point depth buffer always has enhanced precision... but the reason has nothing to do with the fact that you switched to floating-point. Depth values are generally already in the range 0.0 - 1.0 whether you use floating-point or not, so by themselves nothing is gained.
An interesting property of floating-point depth buffers, however, is that if you invert the depth range and use a floating-point depth buffer you can often improve the consistency in precision near and far. Typically because of the way perspective projection works precision is distributed with a bias toward points close to the near plane. Emil Persson has a great discussion of the situation here. He also has a few articles to his name on the subject while he worked at AMD, though they are less pertinent.
Also, you should never linearize the depth buffer. This requires writing a new value to the depth buffer after rasterization, and will prevent hardware optimizations such as Hierarchical Z-Buffering and Early Depth Tests from functioning as intended. Do everything you can to maximize the precision of the non-linear depth buffer, rather than fundamentally altering what is stored in the depth buffer.
As a capstone to this entire disucssion, you should consider the following article, because it discusses the ramifications of all of the representations I mentioned above in terms of re-constructing position from depth. The takeaway point is that inverse perspective depth (1 – Z/W, 32-bit floating point) is nearly as good as linear depth, and it still benefits from hardware Z-buffer optimizations.
