After reading tons of reviews and documentation on "Crossfire" I believe that it is a much better dual card solution. Although more expensive, the performance increase over SLI is amazing.
Nubius was right, 1-Card is the master - one is the slave. The connect with a special DVI cable on the back of the PC. Also, you need a special edition "Crossfire Enabled" MOBO. But, you need a Special SLI board for nVIDIA's solution, so it's the same.
I'd love to see the benchmarks of (2) X900's in crossfire vs. (2) 7800GTX's in SLI. IT's going to be really interesting to see which company has outdone the other.
My money is on ATI this round...
LOL - I woundn't be suprised if after R520's are launched, nVIDIA launches a 7800GTX ultra (32 pipelines with 512MB) something like that.