Re: XvMC Quality?
The geforce 6200 (the same for 6600 and 7800) doesn't support hardware video overlay this means it doesn't do Xv using overlay in hardware anymore. The drivers use simple blitting (a simple 2d operation).
The image quality might improve if you let XvMC use the 3d engine of your GPU instead of Xv. Add the line Option "XvmcUsesTextures" to the screen/device section of your X config file. Perhaps it helps.