This move (going to the D800 and keep using DX lenses ) would make sense only if you plan to replace them with FX lenses, at least some of them, in the near future. Have you looked thru the viewfinder of an entry level DSLR, like the D3100? If not, you should, before you go forward with this plan. Because the tiny viewfinder of that camera is considerably larger than the DX portion of the D800 viewfinder. People may have different tolerance for this, but I would not get any pleasure from shooting thru such tunnel vision. And since photography is a hobby for me, it is about pleasure.
So: if you want to keep your DX lenses and want to upgrade the body, I'd suggest looking for a DX camera. Otherwise you may find that you will have to replace your DX lenses (or get sell them and use just your FX glass).