what is the recommanded power supply for 2 4870x2 lol
I would go for at least an 850w. I run my 2x 4870x2 with an Antec truepower quattro 1000w
and I use every bit of it but I have plenty of other components in my system that suck a lot of juice also so YMMV.
but the 4870x2 doesnt support dx11... its important difference on graphic? lol
You'll be absolutely fine. The change from DirectX10 to DirectX11 was evolutionary rather than revolutionary, and as such, backward compatibility is relatively trivial and built into DirectX 11 directly:
You will be able to run a game, using the game's DirectX11 codepath, directly on DirectX10.1 hardware (such as the 4870x2). You will be missing out on a few DirectX11-only features such as tessellation, and you will be using shader model 4.1 instead of 5.0, but your experience should not be too far off.
In fact, many of the pioneering advancements that DirectX10.1 introduced over 10.0 were never widely used at the time, but were ultimately rolled into DirectX11. So as odd as it might sound, it is via the 10.1 downlevel compatibility path in DirectX11 that many of us with 10.1 cards will finally be making full use of our hardware.
My experience with Quad Crossfire has been very good. In Bad Company 2 I get better FPS than my friend who has a 5970 and an amazing amount of games work great with quad crossfire, including old games that you wouldn't even expect to work such as World of Warcraft and Source engine based games. ATI now releases crossfire application profiles separate from the drivers allowing you to keep your crossfire compatibility at top notch without having to constantly install new drivers.
If you were starting from scratch I would probably recommend going another route, but it's going to be impossible to beat the performance you will be able to get by adding another 4870x2 for anywhere near the price.