Right now, the only overall upgrade for the D300 is the D7100. The D300 has the "pro" body ergonomics, a dedicated AF-On button, and a higher frame rate than the D7100. It's also bigger and heavier. But, those things don't technically improve your photos.
In terms of image quality, the D7100 (and the D7000 for that matter) exceeds the D300 in every way, especially in bad light.
That said, if pure image quality is your only concern, a D3200 will give you better dynamic range, more resolution, and better high ISO performance than a D300 or D300s. You'll be sorely lacking in AF performance, frame rate, and controls, though. I'm somewhat joking, since the D3200 is not a practical upgrade from a D300.
You really have two choices if you want to stay with DX: buy a D7100 or wait for the chimera D400 to come out.
If I only wanted to use mf lenses would I be better off with the d3200? My biggest reason for looking into a d300 is the 100% viewfinder. Based on these factors is the 3200 still not a practical upgrade?
This thread is going to get complicated to follow since there are two questions, with different requirements, active. Regarding the DX companion to the D3s, for IQ, low light, resolution, DR, and AF, the D7100 is the best option if you do not mind switching back and forth to different control layouts. I shoot with both a D800 and D7000 and it is all automatic for me, never thing about the differences, but differences there are. Some users are very sensitive to layout consistency and it takes precedence over IQ. If you have more than one car, the differences between car control layouts are not that important to you I would assume.
Regarding the MF options: The D3200 has a small VF making MF a lot harder. Pick a camera that has a large VF, 100% is not essential, like a used D700 has a large bright VF but not 100%. An important note to be aware of is Katzeyes, the only readily available focusing screens has lowered coverage of Nikon's nearer cameras like the D800 and D600. A focusing screen on the D300 would be easy to focus MF with, a D700 even better If you are a tripod shooter, LiveView on all the newer cameras make VF manual focusing less of an issue. I have a new 50 1.2mf(yes they still make them and it is a joy to hold and feel) and it is almost impossible to MF on the D7000 at f/1.2 but not bad on the D800 without a 3rd party focusing screen, but both do very well with LV. Stan St Petersburg Russia
>If I only wanted to use mf lenses would I be better off with >the d3200? My biggest reason for looking into a d300 is the >100% viewfinder. Based on these factors is the 3200 still not >a practical upgrade?
By mf lenses if you mean lenses without a CPU then the D3200 will not meter with these lenses or autofocus with the older AF lenses. The D300 will meter with mf lenses but so will the D7000 and D7100 which have better high ISO performance, are higher resolution and are more versatile than the D300. The best DX body on the market right now is the D7100.