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The D700 long in the tooth? D700 is alright?! Oh, how the mighty have fallen!
You will see a marked improvement in image quality by using good, fast glass on a D700 vs an 18-200 on the D300s. With fast glass and a D700, you will be able to shoot at faster shutter speeds, and in lower light. The D700 will give you better dynamic range, too.
I've made 16x20", 24x20", and 20x30" enlargements from 6, 10, and 12 megapixel shots from a D40, D200, and D90/D300/D700... if you can't get a good 20x30" print from those cameras, the problem isn't the camera or lens. Hell, I've printed 16x20s from D2H (4.1 MP) shots.
All that said, you'll see a marked improvement in your images on your existing D300s if you use faster, sharper lenses than the 18-200. There are excellent ultra-wide lenses like the Tokina 11-16/2.8, the Nikon 12-24/4, etc. There are excellent primes in the normal and telephoto ranges that are FX lenses as well, should you decide to switch to FX later. There are tons of great, and (compared to FX equivalents) inexpensive f/2.8 zooms that are razor sharp, like the 17-55/2.8 AFS, the Tamron 17-50/2.8, Tokina 16-50/2.8, etc etc.
What exactly ISN'T your D300s doing that you need improved? Is it the look of FX's shallower depth of focus vs. DX? Is it the low-light performance after ISO 1600?
I will say this: unless you're printing billboards, the D800's resolution will be lost on you, and you will be regularly buying hard drives to hold the massive files. The D800's dynamic range is amazing, however.