Thanks Jeremiah...I don't have my heart set on any motorcycle yet, just asking some opinions and investigating. What you said about the YZ450F is what I figured. I loved my 99 YZ400F on a motocross track, or even for fast cross-country style woods riding, but I didn't like it in the tight stuff. I assumed the new 450 would much the same, perhaps worse, but wasn't sure since I haven't ridden one.
As for a new 300 EXC, I haven't ruled out that possibility, but I already have a 300 KTM that I've been riding for the last three years, and I love my motorcycle but I'm ready for something different. There are a lot of things I adore about my 300, but I don't care for the suspension or the steering, and more than anything else, I'm just ready for a change next time. Also, I'm not happy with the way my 300 performs on a motocross track. That's not a huge deal, as most of my riding is in the trees, but I want more versatility on the next machine.
Why wouldn't you call a 450 a "big thumper?" In my mind, there are two basic sizes of four-strokes on the market today...450s and 250s with the 450 being the "bigger" of the two. Sure, Husaberg and KTM make a bigger bike, and there is the XR650 Honda, but basically 450cc is the bigger version. I realize there are bigger machines available and when I raced dirt track, I rode a lot of 600cc Rotax's and XR750 Harley's, so by those comparisons, you're right, the 450s aren't big thumpers. But for modern motocross bikes, they are. Anything bigger than the Honda or Yamaha 450 wouldn't be practical for my application.