Is that confusing? I'm telling him the keep his video card and just buy a compatible motherboard for dual video cards. Also he'll need a power supply marked for it too (SLI certified).
When the "price is right" he can buy another 7800GT for cheap. 7800GT's SLI'd perform outstandly in most games. More so than a single 7800GTX by a 10-20% margin in frames per second when the graphics get cranked.
So overall don't scrap the system because what you want is already there except a few minor compatability issues and the aquiring of a 2nd 7800GT...which is cheaper and more convient than selling and build from the ground up practically the same system + another video card
...You want a 7800GTX SLI BEAST. Which would be 110% boneriffic.
just my 2cents