Windows 2000 or better is required.
An ATI Radeon 9800 is used for development, so that card will always work. Mediocre has been tested on a Radeon 9600, but not recently. Quadro FX 500 works.
Strictly speaking, Mediocre will work on just about any graphics card, it may just perform badly. The more features the graphics driver exposes through OpenGL, the better Mediocre will perform. It uses programmable hardware to speed up the decoding process and to provide graphical effects.
These are the cards that are known to work in full hardware mode. If you know about more working configurations or have experienced problems, please leave a comment.
If your graphics card does not have the required features more work will be handed to DirectShow, which in turn will require filters to actually do it. Filters (usually) run on the CPU, so this will slow down the decoding process.
The “big three”—nVidia, ATI, and 3dlabs are alone in providing satisfactory support for OpenGL. Their recent cards all support the required features.