two 4870x2's is overkill and a waste of money and you will not get any performance increase from using 2 of these.

one 4870x2 is the same as 2x 4870's(speaking of performance). not sure how the 4870x2 handles running the 2 gpu's on one card, with regards to xfire tech. but if given the choice between one 4870x2 or 2x 4870's than I would go with the 2x 4870's. the 4870x2 is a huge video card, (3lbs and 12 inches long)I think it is an accident waiting to happen. also if one of your 4870's goes bad you still have one card to use, if anything goes on the 4870x2 than you got nothing.

sli/crossfire is just a plastic connector that links the two video cards together. snaps in place in about 10 seconds, change your software settings and your done. If you plan to multibox than you will not get much benefit from using xfire. if you plan to use only 2 monitors than you can do it with just one 260/280 or 4850/4870. if you plan to use a 3 monitor setup than you will need 2 video cards, but don't link them together in sli/xfire mode unless you want to use the matrox 3heads2go. the 3heads2go with 3 monitors is pretty cool and gives almost 180 degree vision for a more realistic gaming experience.