If the rumour is true about the 50D then the spec listed her still falls short of the D300, which is an amazing camera. A friend of mine has the D300 and the results are great. In low light it is much better than my 400D. Though my 400D is not any ware near the D300's league is really is a great DSLR !
I know you've said, but you cannot EVER compare the 400d with the d300.
it's like comparing a 40d to a d40.
!