ATI Radeon HD 4870 X2 Vs. ATI Radeon HD 4870 CrossFire Review
 

It is at a resolution of 2560x1600 that the Radeon HD 4870 X2 is brilliant whereas the Radeon HD 4870 CrossFire is not, because it has only 512 megabytes of graphics memory. We guess two HD 4870 cards with 1GB of memory on each would behave exactly like the 4870 X2.

Anyway, the Radeon HD 4870 CrossFire beats the GeForce GTX 280 in nine tests and loses to it in only three out of 15 tests that support resolutions above 1920x1200. Moreover, the CrossFire tandem delivered a playable frame rate in most of the tests.

http://www.xbitlabs.com/articles/vid...crossfire.html