I'm just getting into Real Estate photography, I'm currently using a D3100 that seems to work well for me so far. I have been told I really need to move up to a FX camera body. Ok I understand the sensor size, but really what am would I gain? It is a pretty good chunk of money a good FX body.
Indeed, this has been discussed, but relevant answer is pretty deep in there.
I think for real estate, when you want rooms and house to look As Big As Possible, Full-Frame's benefit is a possibility of using wider lenses. For example, Nikon's 14mm lens has a field of view that would require non-existing 9.3mm lens to reproduce on crop sensor (d3100)
Canon has full-frame 11mm lens that produces same field of view as non-existing 6.9mm lens on crop sensor
PS: as far as I know, widest crop lens for Canon or Nikon is Sigma 8-16mm zoom, that is equivalent to 12-24mm on full-frame. You might want to give it a try.
PPS: Here we talk only about rectilinear lenses, not fisheye that can come in wider focal lengths but will create great distortion to the image (mostly for artistic purposes)