I've had a couple of Nvidia cards, all with the same result: you either choose the monitor or TV output. With TVtool I've managed to get a mirrored/dual display, but the refresh rate of the monitor has to be set so low to be compatible with the TV (you can only set a uniform rate), that the display is quite frankly awful.
Added to this, the screen is merely mirrored, so it doesn't really help for preview! For true suppory you really need a card from Matrox:
http://www.videomaker.com/scripts/ar...=9583&GU=le929