Nikon D200 has a 10.0MP APS-C (23.6 x 15.8 mm ) sized CCD sensor . On the other hand, Nikon D300 has a 12.0MP APS-C (23.6 x 15.8 mm ) sized CMOS sensor and features Expeed processor.
Below you can see the D200 and D300 sensor size comparison.
Nikon D200 and Nikon D300 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Nikon D200 has 21% larger pixel area compared to Nikon D300, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.