I have several MPEG decoder cards/addons for various video cards:

Creative DXR2/PC-DVD/CT7220

Diamond MVP 1100/S3 Scenic MX2

(not my photo, but I have the exact same S3 Trio/daughterboard combo)

Matrox Millenium with an addon card

Cirrus Logic CL-GD5446 with a TV tuner (not sure if it has MPEG, though)

Now I know that some games had special editions for Reelmagic MPEG1 cards that offered videos in higher quality/resolution/size. Could later MPEG cards be used to enhance, say, FMV games? Which titles supported that? What kind of hardware/software setup did you need for that?