Snip: X1900 Crossfire is currently the fastest, most feature-rich, highest image quality solution I’ve ever tested. The performance of two X1900s is simply mind-blowing when a game works with it properly. NVIDIA has a solution with four GPUs, Quad-SLI but this isn’t on the market in any numbers as of yet and will cost the buyer 2x or more a Crossfire setup. X1800 Crossfire is shown here simply to give a sense of performance if you buy a second X1800 card. X1600 Crossfire is an interesting choice in that the cards can work without a Crossfire cable.
Full review @ Motherboards.org