Hi DarkPoet,
While it's quite possible to do what you plan on doing, I'd urge against it as most integrated videocards also use shared memory with your system. This slows down overall system performance as now your CPU needs to arbitrate use of it's system memory... even if the card is just sitting on a plain desktop with nothing occurring.
Moreover, most 6600GT's already have support for dual-head/dual-display, so for your purposes you really shouldn't have to enable to the on-board video to get your dual-monitor configuration working.
Inspect the back of your 6600GT to ensure it has both a VGA and DVI connector. There are cheap (and usually included) DVI->VGA adapters that will allow you to configure a secondary display and use the NVidia drivers (Nview is part of Forceware and very nice/easy to use) to configure the second display however you want (extended desktop, clone primary, overlay, full secondary, etc.etc.)