As noted, you could by a D3X and it won't make your images look any better at your current skill level. Based on your comments, it appears you think it's your gear holding you back and it's not. Don't fall into the trap of thinking you can improve your photography if you throw enough money at it.
I would focus less on buying a new body right now and focus more on trying to figure out how to take (and edit) a quality photo. Your current body will take stunning pictures if you do your part.
Equipment won't make you a better photographer.
With that said, when you do step up to full frame you have several good choices.
The D700 is a fine choice, but it's not necessarily the best portraiture camera. "Best" is very subjective. If we went by popularity and sales numbers, then the 5D2 would be the "best", if not the most popular, full frame camera.
The AF system of the D700 is great for shooting sports but really has little advantage in the studio. The 5D2 has 21 megapixels for fine detail whereas the D700 has 12 megapixels. To get 20+ megapixels in the Nikon line you have to spend $8,000 on a D3X.
Canon also has top of the line glass that is typically more affordable than the Nikon equivalent. Your camera is only as good as the glass you attach to it, and if you want a comprehensive kit you can get more bang for your buck with Canon.
I'm not trying to push you towards Canon. Some folks think Nikon is the end-all, be-all and honestly, it's not. There are Canon folks that think Canon is the end-all, be-all as well, and it's not.
Don't put too much stock in what people say about their pet brands on the internet. Get out and handle the cameras you might be interested in. Rent them, shoot with them at the local store, then make an informed purchasing decision based on the info you gather. Price things out, look down the road to future purchases... check out things like after sales support. I've found that the Canon CPS program is amazing as an example.
But for now, focus on learning how to use a camera then worry about upgrading to a new body/system. Once you figure out how to use a camera properly, you may discover your tastes/needs have changed.