Digital upgrade path

If you don't know you need FX, you probably don't... You'll probably be insanely happy with a D300 and DX lenses.

Scott,
I used a D70 before upgrading to a D300. I am very happy with both.

Whether you should buy a D700 vs. a D300, it all depends on what you are shooting. For me, the final desicion was between longer reach in sports photos (so I did not have to buy extra lenses) (D300) or the High ISO capabilities of the D700.

Being a cheapskate, I went with the D300, as I did not have that much use for the extra High ISO capabilities (>3200 ISO), and that I could use my other lenses I previously used on the D70.
Whether the FX is the wave of the future? Only time will tell, but Nikon has sold so much DX cameras and lenses, so that they will not stop developing and manufacturing them over night.

Remember that you make an investment in the lenses, and not in the cameras. Cameras come and go. Are you prepared to make an investment (or cash out if you are not a selling pro) in FX lenses. If not, I suggest you stay with the DX lenses, which really are not bad.

Difficult to say. I agree with Peter above in general. Probably... I don't know how much your budget is either.
Having said that, I did upgrade from D80 to D700 and quite happy with it. But be fore-warned that you will want FX lenses and it will cost you far more $$$ (can be substantially retrieved when you sell them, though). But I can't say whether I would NOT be happy with D300 because I never tried that path.
So, it really depends how badly you need/want to upgrade and how much "fund" you can commit to the FX path. Good luck and enjoy.

I also use a Mamiya RZ-67 MF and was contemplating a digital back for it, but the prices are more than a New D700. I have read that the new FX format gives you images close to the quality of MF even though the format is smaller. This is primarily the reason I was looking at the FX format.

If you don't have a specific reasoning for the D700 you probably don't need it. Don't assume you're going to need FX eventually because DX will disappear, because all indications are that's not true as Nikon continues to announce new DX cameras and lenses.

BTW if you like the D70 but want the newest technology, what you want is a D90.

Keep in mind that DX lens on a D700 not only crop as your D70 does, it also drops from 12 to 5 Megapixel. I can't imagine why you would want to buy a near $3000 camera to shoot 5 megapixels given that a D300 has essentially the same capabiliities.

budget and future plans. If your familar with the DX format, and set up with a DX lens or 2, theres no reason to go FX. The D90 and D300 are excellent cameras. If Upgrade is your sole reason for getting a new camera then go for the D300, it has the more advanced features.
If you have a lot of prime lenes like me both manual and auto focus, an FX body is not a bad idea if you can afford it get a D700.

>> "Is the new FX format the wave of the future and is it worth the extra cost?"

The FX format allows superior performance for certain things, but the DX format still has lots of potential and is perfectly good for almost everything. In the future both formats will probably exist side by side.

>> "I understand that the D700 will use my DX lenses, but will just crop as my D70 does."

Using DX lenses on a D700 is not exactly the best thing to do. You'll end up cropping the images. DX lenses obviously work better on DX bodies than on FX bodies set in DX crop mode.

If you're heavily invested in DX glasses, and don't need the best available high ISO performance, you should be fine sticking with DX bodies. The D300 is a good pro-grade choice. But do also look at the D90 as a lower cost alternative; the "upcoming" D300 could also be a feasible choice like the original D300, with a few extra features.