You have to decide how much you value each difference. How much would each benefit you and the type of photography you do. Instead of focusing (pardon the pun) on how competitive it is, evaluate how it helps you get the images you want. If your tool lets you do what you want, it doesn't matter as much what else is out there.
Maybe if you can say, "Now matter how much I try, the 30D isn't letting me quite get what I want, and features X and Y on the D300 seem like they would help me much more than anything I see on the 40D."
It will be worth it for some to switch, not worth it to others. From what I can tell - and again this is something everyone has to decide for themselves, IQ and high ISO are about the same depending on subject and conditions. I have seen a number of very good shots from the new Nikons and Canons.
If you are primarily a sports shooter or otherwise a high action shooter, you should definitely consider the D300 - with its 51-point AF it is likely much better for tracking a subject, though the 40D has shown to perform very well there. I haven't seen D300 tracking samples yet.
I don't think the advantage are so clearcut otherwise. It will more breakdown to personal preferences for specs and ergonomics.
In my opinion, $500 is a lot of cash relative to the overall prices, and for me the D300 is at best, worth $500 more, and frankly (this is just what they are worth to me), I'd put it closer to around $300-$350.
But then, I have always preferred Canon's ergonomics. With the new changes, especially the Custom dial settings and My Menu, the 40D really suits me much better. And with sRAW, Canon may have not been listening to me, but they appear to have been listening to some similarly minded photographers.