Hi, I know it is possible, but you must setup /etc/X11/xorg.conf to enable it.
You can also use them in SLI mode or just use the onboard for physx and the pcie card for main processing.
Unfortunately, I have not done it.
You should be able to enable both chips, but boot to the pcie vga. the onboard will be used for physx (slaved).
My Motherboard is also marked as an gf8200 although it does not have a graphics chip on board (MCP78).
You may find more concise info at nvnews forums.
In particular the linux section, http://www.nvnews.net/vbulletin/forumdisplay.php?f=13
Hope this gives you some where to look, while you are waiting for a more concise answer from a guru here at LQ.