Regarde ici,
http://www.techreport.com/reviews/2005q4/r...re/index.x?pg=1
the Radeon X1800 CrossFire Edition can also work in concert with the Radeon X1800 XL, a lower end card with 500MHz GPU clock and only 256MB of memory clocked at 500MHz (or 1GHz effective). In fact, ATI has no plans to release a slower, less expensive CrossFire card to match up more exactly with the Radeon X1800 XL. If you want to do the dually thing with your X1800 XL, you're gonna have to pony up for the full, six-hundred-dollar Radeon X1800 CrossFire experience. Then, once you plop the CrossFire card into your system next to the Radeon X1800 XL, the CrossFire card will proceed to deactivate half of its 512MB memory in order to match up with the XL's RAM size. (The Radeon X1800 CrossFire will maintain its higher GPU and memory clock speeds, though.)