After a few months in repair, my D50 finally came back working perfectly. This allowed me to compare the two cameras. One thing I noted is that the exposure meter is far more intelligent in the D300 than it is in the D50. The latter regularly underexposes by a full stop if you let it do what it wants. Another thing is resolution. Since the D50 is 6 MP and the D300 is 12 MP you would expect a considerable increase in resolution. It is a question how significant this is of course as the linear resolution increase is only √2. So I shot a few realistic images with sharp lenses using a tripod in the same location and same lighting and same exposure.
Here is the overview:
At this scale there is no difference whatsoever between the cameras when using Lightroom to develop the RAW except a small difference in color, so I didn't bother to put both up. Below is a detail. Left = D50, right = D300. Both are using default capture sharpening in Lightroom. Both can benefit about equally from some finetuning of the capture sharpening but it doesn't impact the comparison really.
And here is the D50 image scaled up to match the D300 image:
I did all cropping and scaling in Photoshop to avoid the very annoying bug in Lightroom that causes halos around strong contrast edges that even shows up when all you do is just cropping. The bottom line is far more detail in the D300 image and this is the cheap kit lens that came with the D50. Of course the image you see is a detail of a blowup of 42"x28" (I assume your screen is 100 ppi) for the D300 case and 30"x20" for the D50 case. When printed at something like 8x12, you will not see any difference between the cameras, except perhaps with a magnifying glass. These differences are smaller than you'd think at normal print sizes. Also, technique is far more important than sensor resolution in general. No sensor is going to save you from motion blur, unsteady hands, or incorrect focus. Most lenses, even cheap ones, are plenty sharp when used correctly. All a camera like the D300 does is focus faster and more consistent, expose better, and in general get out of your way more. I love my D50 for how light and simple it is and if you do not need the better framerate or better focusing, something like a D40 will work fine for you.