First of all always buy fast good lenses first as you have been told here already. Then if there is something about your camera that it doesn’t do for you then look at a different body.
I used a D80 for years coming from a film camera but didn’t like the low light capabilities and noise from it, so I started to look for a new camera. Although I had FX lenses already from my film days, and some manual lenses, I was ready to go to a FX body. I always wanted the D3X but it was very expensive. The D800 came out at the time I was ready to buy and I read everything I could about it. I finally bought one and just love it, it takes really great pictures and all I could ever want in a camera. It really pulls in the detail in pictures and is just amazing. Every lens I put on it makes that lens better, although I have never shot with a DX lens on it yet. But then again I like to print 20X30 prints that the D80 can’t compare to the D800. But I have prints from my D80 that I upsized and have printed that I wished I had the D800 for all the detail. But if you don’t print big prints, your camera is just fine you won’t see much difference.
On the down side is it is less forgiving and you need to be on your toes using your best shooting practices. The files are very large coming out of the camera and the need for larger cards in camera and a two TB hard drive for your computer. Also I have noticed that if you shoot RAW it takes a little more time to download and process your picture files. I use Nikon’s Capture NX2 and only load 1 file at a time with my D800, where I used to load 10 to 12 at a time with my D80 into the software to work on.
I still have my D80 and still use it as it is easy and fast to shoot, where the D800 still takes me a little more time to click the shutter. But I am getting better every day and hope to be as fast next year. But I would never have looked for the D800 if not for the noise and low light capabilities of the D80. So until your body can’t do what you want it to do stay with what you have and buy good fast glass that will last you a lifetime, body’s come and go. If you are thinking of going to a FX body in the future then start buying FX lenses now so you will have them. I used FX lenses on my D80 all the time and still do.