It is technically true, it's called hybrid crossfire, but only worked on really low end cards like a 3450 and showed limited performance gains. Your current board wouldn't be able to do that with new cards, and even the cheapest of new cards would perform better on their own.
AMD now does hybrid crossfire with the new llano series of APUs, they have an integrated video card on the processor itself instead of the motherboard, and can crossfire with Radeon 6000 series cards up to the 6670. Once again lower end cards.
To sum it up, it is possible, but not worth the cost and would give horrible gaming performance. I'm not sure what your current Nvidia card is, but it'll probably be slower.