To be fair to the D7100 it is superior technology to the D300. You lose buffer for sure and the familiar layout, but you gain quite a bit of dynamic range, and you get better ISO performance with 2x as many MPs. The photos being posted here on the D7000/7100 forum are stunning. You gain the ability to crop significantly and maintain usable photos. The AF system though it has less AF points/VF coverage is a new generation module and gets great reviews.
That being said I own a D700 and I love it so much, that when I added a D800e I refused to sell my D700. I pull it out mostly for impromptu photo sessions - but to be fair the D800e is the D7100 to its D300 and the improvements in DR and ISO performance see me using the D800e most of the time.
My advice would be if you go FX, go FX. As in get FX lenses and go all in. If you can't afford to I would hold off. I would have no interest to shoot my D700 in crop mode, in fact I never have. I sold my entire DX kit when I moved to FX and never looked back. The D600 and D800/e cameras are amazing. That being said, if you don't need the buffer and the form factor is satisfactory the D7100 holds its own amongst the new class of Nikons. You would have to change cards, good SD cards can be pretty cheap though.
Another option is to wait it out a little more and see if Nikon ever does release the D400. It bears noting that despite constant rumors over the last 3 years that it was imminent, it has never materialized. Still if you are not in a hurry, you could wait a bit, worst case scenario you still have your money and the D300 is a very capable camera.