Nikon D300 has a 12.0MP APS-C (23.6 x 15.8 mm ) sized CMOS sensor and features Expeed processor. On the other hand, Nikon D200 has a 10.0MP APS-C (23.6 x 15.8 mm ) sized CCD sensor .
Below you can see the D300 and D200 sensor size comparison.
Nikon D300 and Nikon D200 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 (37.14µm2 vs 30.54µm2) compared to Nikon D300, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.