You can check compatibility yourself:
1. Go to this page to see the different releases: http://forums.adobe.com/thread/311515?tstart=0
2. Navigate into the download link for the appropriate version of Camera Raw.
3. Follow the link to the ReadMe (e.g., http://www.adobe.com/special/photoshop/camera_raw/Camera_Raw_4.6_ReadMe.pdf).
4. Look to see whether your camera is supported.
Photoshop CS6 is anticipated to be out in a few months, and Camera Raw is supposed to have a whole new approach, so a lot of us are excited about anticipating that.
According to the third question on the FAQ page Noel linked to, http://kb2.adobe.com/cps/407/kb407111.html, the D300s was first supported by ACR 5.5, that works with CS4.
There is no need to upgrade, you can use the DNG converter on the files, so that they can be opened by 4.6 (but the new engine of ACR 6 (Ps CS5) is too good to be missed... But as CS6 is around the corner... I'd say to wait a little bit.