"So if I get a video card, would I be able to run my 42" monitor off of that and then the dual 22"s off the on-board video?"
As torrentg said, yes you can quite easily. My motherboard has the identical chipset as yours (NB780G/SB700) and do just that,
The one on the left is my 32" Westinghouse TV connected to the onboard HD3200 (and I don't get the lag you're describing), and the other two (22" and 19" Acer LCD's) are connected to my HD4830. To do it is just a matter of enabling the SurroundView option in the bios, that and only works if the video card is an ATI card. From your motherboard manual (mine says the same thing),
This option is configurable only when Init Display First is set to PEG. Enables or disables the
Surround View function. This option is configurable only if an ATI graphics card is installed.