I would just upgrade the card personally. Replacing a motherboard is a much more complicated procedure than replacing a video card and usually requires reinstalling Windows.
Something like this;http://www.newegg.com/Product/Product.aspx?Item=N82E16814125334
Is more powerful than SLIed 8800GTs, will have no SLI scaling/compatibility issues, can also overclock a very large amount(usually 30%+), has more current features like DX11 and is much more power efficient, especially at idle. If you ebay the 8800GT it will probably bring the total cost down to approximately what you would spend on an SLI motherboard anyway.