I believe it is the $600 camera buyer that has the biggest problem. The sensor does not care how much you paid for it, and now the lowest entry level Nikon DSLR has the highest pixel density in the entire line, and only the D800 exceeds it in pixel count.
A strange situation indeed since pixel density and pixel count determine the relative need for absolute stability. (equalizing field of view, it is actually the pixel count, not density that determines the relative need for stability but the two tend to go hand in hand).
Any high end head can be attached to any tripod with a removable head and a 1/4" or 3/8" stud on the tripod platform. There are some very inexpensive combos that don't appear to follow that standard.
To answer your question as to which to upgrade first, I think you have to carefully define your problem(s). You should have a problem before spending money to upgrade gear. Otherwise you'd be spending just to keep up with the Joneses or bragging rights.
As far as horses and saddles... a basic Manfrotto 055 will go a long way, until you get into long focal length shooting (past 200mm or so) where tripods are just as important as the lenses. But that compromises creature comforts (carrying weight per unit of stability) and that is what starts the upgrade spiral as you start looking at CF tripods, and then a commensurate head.