The safest way would be to replace the power supply. I ran across this model from Seasonic recently and like it in the price range http://www.newegg.com/Product/Product.aspx?item=N82E168...
The second option, that we would not really recommend, is to use an adapter http://www.newegg.com/Product/Product.aspx?Item=N82E168... Normally I would not even suggest that, but I'm sure someone will. This should work, but it has a chance of crashing the system and the video card, and the cables can fail, melting the connection and possibly damaging the PSU or video card.
I'd sell your current PSU on craigslist or somewhere, and get a better one with the 8 pin cable on it already rather than use adapters.