I installed Civil 3D first with no problems except that it blew out 2005 Civil 3D (no real problem as that barely worked anyway) and didn't get around to LDDT 2006 until last month. That went: LDD, Civil Design and Survey (you will need all 3 to get a fully functional product as there is no hydrology or survey modules for Civil 3D and the piping module is pretty restricted partwise).
I have SP1 for Civil 3D installed (it is absolutely necessary for stability issues). There is also a SP1 for LDD as well as SP2 and a hotfix for Civil 3D that I have not installed. A friend had nothing but trouble installing the LDD service pack (2 reinstalls and intense registry repairs were required for him to get back to SP1). Since I don't really like or use the LDD very much and as it is functioning now I have seen no need to risk my install. SP2 for Civil 3d and the hotfix require the LDD service pack installed first and since that program has become stable I haven't applied these either.
If you intend to use Civil 3D, get ready for a big old ornery bull ride. Points management and cogo functions seem to work OK, alignments, corridor modeling and profiles are ready for prime time and it can produce great surfaces, but I have found parcels and especially their labeling to be a disappointment and the grading is still notoriously unstable until you figure out exactly what the program does not do - yet. A lot of familiar LDD tools will seem to be missing, but most are hiding under different names or have been replaced.
Your biggest hurdle will be the program interface. After you have fought with it long enough you will begin to see that it does work, is extremely powerful and that it can be mastered, but the same can be said about the old Edlin text line editor and this puppy is almost as obtuse. This program is most definitely NOT one to try learning on your own. Get the best training and as much of it as you can afford. I got my training from an instructor independent from Autodesk and their reseller network. Feel free to contact me if you would like a referal.
