Two Graphics Cards?

I think i've seen somewhere that you can connect two graphics cards together to work in parallel. Dramatically increase your graphics capabilities, or something.

Is there truth in this? If so, any good articles? Links? How-To's?

>>1
I call it waste of money on PCI Express. Today's graphic cards are good enough, and if a 6600 isn't fast enough, get a 6800. By the time it's slow and you want more, two 6800 won't be a desirable solution, as there'll be new shader models and features in newer cards which you also want, besides raw speed. So no, it's not a good idea, it's a waste of money.