ExtremeTech compares ATI's Radeon X1800 XT with NVIDIA's GeForce 7800 GTX with the latest official drivers. In this review they don't take a look at gaming performance but they check out video quality and performance (CPU utilization in this case).
Both Nvidia and ATI tout superior visual quality to each other, with marketing side-by-side comparisons demonstrating superior de-interlacing, de-blocking, color vibrancy, scaling—you name it. In many ways, video quality is entirely subjective. There is one good objective benchmark of video quality we like to use, and that is Silicon Optics' HQV Benchmark DVD. The HQV DVD includes a series of tests that are meant to duplicate common challenges in broadcast video (and some DVDs), including de-interlacing quality, detail and sharpening, noise reduction, combined scrolling text and video, and various film cadence patterns..