I've had nVidia/Matrox, nVidia/3dfx (Voodoo 3), 3dfx/Matrox, Matrox/Cirrus Logic, and all have been perfectly fine, no problems at all. Unless you've got a dual head card (like my Ti4200), then overlay will only be on the primary monitor, for DVDs/videos/games, but you can display anything 2d on the 2nd monitor 🙂
Confused