A D3X will limit you less than a rebel.
For example: Low light photos.
In this case a D3X will make your photos look better and chances are that with the rebel you'll be worrying about noise more than composition.
A good photographer is someone who takes good photos. Good photos have good composition and good image quality.
The ideal camera is the one that makes you worry more about composition than it's technical limitations. In that sense then yes, a D3X will make your photos look better than if you made them with, for example, a rebel.
A crappy photo could become a mediocre photo and a good photo could become a great photo if you used a D3x instead of a rebel, assuming the D3x has a technical advantage over the rebel in said cases.
The logic is right. That is why no professional photographer uses a cell phone camera as their main camera.
If you are passinate about photography and want to buy a D3x because you believe it will make your photos look better (thus making you a better photographer) then go ahead! It's your money (hopefully) and none of my business.
I just don't think remortgaging your house is a good idea.