[VinnyP]

40D is a great body and incredible value. The Nikon pro zooms perform better (Unless you plan to use your 70-200 on a D3) but Canon definitely to me has as much of an advantage in short fast primes.

The big difference for your criteria is the 40D has never claimed to be a semi pro body as far as I know, no meaningful weather sealing, no Af fine tune not a 100% viewfinder and rated for 50% less shutter actuations. It can produce images plenty good enough for pro use though I have no doubt. I do have doubts about the usefulness of scene modes on a pro DSLR.

D300 is marketed as a pro body. IQ differnces are not meaningful but the D300 will give you nearly 30% more pixels on what you see in the viewfinder that's enough for a good crop whilst still keeping the image saleable. The D300 has the better Af, better flash system, a much better screen, better auto ISO, marginally better high ISO noise, clears the buffer quicker and has a better vertical grip.

The 40D weighs less is full time 14 bit, has a histogram in live view and performs faster than the D300 at 14 bit.