That was just the answers that I received when I said I was going the D200 and 18-200 route last July.
What is going to make the difference is your level of desire to learn and what you put into it. The D300 is a camera that will DEMAND that you learn how to use it right to get the most out of it... otherwise, you won't get anything more than the same kind of snapshots that any $150 point&shoot could do for you.
I went the prosumer path, but then again, I literally threw myself into this the best I could and invested a lot in books, courses and most importantly... time. If you have the same level of enthusiasm, by all means invest in a D300... but please don't put a cheap lens on it (I do NOT consider the 18-200 to be a bad lens... its a mid-level quality lens with a LOT of potential, but it cannot compete with better quality lenses, though I call the 18-200 the ultimate walk-around lens, bar none).
Since that time, I've added to my bag and am somewhere in the $4000+ invested in my setup and love it.
I am a big advocate of poeple doing a TON of research themselves BEFORE they purchase, because no one else can be held responsible for them making a good or bad choice for you. Do your homework, really KNOW if you want a D300 or not and then no matter what you purchase, if you have done things right, purchase with confidence.