pci-e 3.0 is a marketing gimmick for current gen gpus. there isn't a gpu in existence which can max out a pci-e 2.0 x16 slot, let alone a pci-e 3.0 slot. Heck a number of gpus still don't max out a pci-e 1.0x16 slot.
you'll need a new motherboard with the AMD, it looks like from your writing that you expect to need a new motherboard for the intel as well.
If you can get those 660s for less then 200 go for your SLi setup. that said, the 7950 (which is much faster then the 660) can be found for $180 these days, and xfire seems to be mostly fixed on single monitor setups. you won't need 16gb of ram... 8 is fine. if 8 lets you get a better gpu to xfire/sli then get 8. cause nothing will make you use 16. Furthermore, ivy bridge and piledriver can't really benefit from ram clocked that fast. The only thing on a haswell that will use ram that fast is the igpu, which you won't be using anyway so that throws that out... if you are getting that a10, you'll need the fastest ram you can get your hands on... the igpu will love you, not that you'll use it, the cpu will love it too because of the lack of L3 cache.