DX to FX
Unless you really need the extra width I would stay right where you are.
Some good advice, and some I disagree with, already given. but the sentence above sums it up best.
For what my experience is worth - though I don't shoot architecture - I tried a D300 (coming from a D700) for the extra reach/wildlife. I found the D700 quality to be so much better (even when cropped in at x1.5) that I quickly sold on the D300.
That said, the D300 can easily attain 'professional' quality photos if used correctly.