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.