First, click here for a list of graphics cards that support the Crossfire technology. You can then click here for the product page of the X550. Based on those two the X550 series does not support Crossfire, so you'll have to look at either a newer ATI line or at the Nvidia SLI offerings.
I am trying to find out which ATI cards are crossfire configured. I have looked at the website, but the one I,m looking at is not there. I am looking at an X550. Can anyone point me in the right direction? Thanks.