This is also a strong interest of mine, so I'm happy to support your work as much as I can. I can say that of course the NASA data is the best source, but I have found it to be inconsistent in resolution and quality. I have been hoping to work with another forum member, BigBen, on a project to create the best possible, easily used, ready to apply out-of-the-box texture set of Earth data, including color, height, clouds, city lights, specular, water, maybe even bathymmetry, but that project hasn't get been started.
In general you can get good basic results just by using the existing NASA data. But as you found, it works best from higher altitudes. It is inevitable that global data will be limited in resolution because as resolution increases, data size increases, and memory use also increases. So even if you have 64GB of RAM, you will soon be out of memory working with huge 50,000 pixel square images/textures/displacement maps/etc.
As for the clouds, I think the best thing to do is work on a setup where the NASA map is a mask or other modifier for the Density Shader of a Cloud Layer in Terragen 3. This allows you to add procedural detail to the NASA data, which works on a smaller scale and thus at closer camera distances. The trick is in getting the right procedural function scales and patterns to complement your NASA cloud map and the right densities, depth, altitude, etc. as well. It may be best with multiple cloud layers all with varying settings, but using the same or similar variations of the NASA cloud mask.
There are a number of threads around here on the forums where several people have achieved varying degrees of success with Earth renders. It's worth looking around and trying a few different searches to see.
And, as Dune mentioned, there are some more procedurally-oriented full planet scenes and presets available at NWDA, which could help with some aspects:
http://www.nwdastore.com/- Oshyan