The guys over at [H]ard|OCP did a comparison between nVida's GeForce GTX 580 in SLI against the AMD Radeon HD 6990 with Radeon HD 6970 in CrossFireX called TriFire.
Check out the findings here:
http://www.hardocp.com/article/2011/04/11/amd_radeon_69906970_crossfirex_trifire_review/1