ixbt put up a nice analysis:
http://www.ixbt-labs.com/articles2/gffx/nv40-rx800-3.html
Their conclusions:
"We can draw the following intermediate conclusions basing on the statements above: (a) the percentage ratio of areas in which ATI and NVIDIA use only bilinear filtering during "optimisation" is practically the same;
using optimised trilinear filtering brings about a significant increase in filtering speed;
both manufacturers get approximately the same gain from using optimised trilinear filtering;
ATI images will sometimes be sharper than those of NVIDIA, but in the case of a moving camera, ATI images are more likely to feature moire and dithering that result from under-filtering;
NVIDIA enables the user to enable/disable optimisation on current drivers for NV40;
ATI doesn't provide this opportunity: the driver itself decides about optimisation enabling/disabling. For some texture classes, the current driver version never uses optimisation; for others, the driver enables optimisation when it thinks that less detalised MIP levels are minimised copies of the previous ones. "
http://www.ixbt-labs.com/articles2/gffx/nv40-rx800-3.html
Their conclusions:
"We can draw the following intermediate conclusions basing on the statements above: (a) the percentage ratio of areas in which ATI and NVIDIA use only bilinear filtering during "optimisation" is practically the same;
using optimised trilinear filtering brings about a significant increase in filtering speed;
both manufacturers get approximately the same gain from using optimised trilinear filtering;
ATI images will sometimes be sharper than those of NVIDIA, but in the case of a moving camera, ATI images are more likely to feature moire and dithering that result from under-filtering;
NVIDIA enables the user to enable/disable optimisation on current drivers for NV40;
ATI doesn't provide this opportunity: the driver itself decides about optimisation enabling/disabling. For some texture classes, the current driver version never uses optimisation; for others, the driver enables optimisation when it thinks that less detalised MIP levels are minimised copies of the previous ones. "