280x gets more updates, and will likely crossfire better. Single card performance is important too as many games tend to have issues with CFX/SLI at launch. The current batch of 280x is just a 7970 rebrand. Supposedly between now and January, Tahiti XTL will emerge.
Most of the games I play do not even support crossfire..
I would much rather have the added support of the 280x than the free games that come with the 7970 haha.
But if you have the motherboard and psu, crossfire will be cheaper. Just some obvious caveats
I had already recognized all those cavets actually
I may not even crossfire a 7870. I may just get one card as I seriously don't play any graphical intensive ones.