you can mix and match modular and hardwired cables with no issues. i'm curious though why you want to go with 2 4870 x2s. you can get much more performance with far less electricity and heat going with newer generation hardware. from what i've seen in benchmarks a single 5870 will beat a 4870 x2. they're roughly the same price I'd get 2 of those an cross fire them, if you can find them in stock.
as for bottle necking on a x8 pcie interface, i woudln't worry much. while i'm sure with the right bench marking you'd be able to find a minor difference mostly due to latency, but it shouldn't be enough to cause any big issues for you. certainly not enough for you to notice in gaming.
1. I can see a small impact form the x8 slots being these cards are dual GPU.
2. That will run them just fine
3. yes that should work.
Now my turn. Please take these suggestions and constructive criticism.
A) Why the 4870x2. They have been replaced in terms of the big daddy cards by ATI. The 5970 (aka 5870x2) is it's replacement and just knocks the 4870x2 out of the park. The single GPU 5870 outperforms the 4870x2 in most cases. If I were you, just get a single 5970, it will run as good as a dual 4870x2 setup, and consume FAR less power. Your setup will have heat issues, unless you have a FULL ATX case with LOTS of fans. TRUST ME they run HOT HOT HOT.
B) Are you limited in terms of location or where you can purchase them? Do you live in the US or another country. Where will you shop to get your equipment?
C) Why so much GPU power? Are you planning to run multiple monitors or super high resolutions? If you are running a single 1920x1200 monitor (in your signature) a single 5870 would do you just FINE. Trust me I have one and it run most everything maxed. At the very most I would get a single 5970.
Honestly I'd take the single 5870 over the two of those. But that is just me. You will have the raw power, but other than Crysis, what other game demands that power? Honestly I played the single player of Crysis, when I beat it, I never picked it up again. But its your deal.