If you get a 5870 it will be a long time before you need a different card, by then Nvidia will likely have come out with new cards and likely much more efficient, so the 850W is probably still excessive.
Also, if it is mainly a gaming computer personally I would drop to an i5 750, P55 mobo and Dual channel RAM to save some money. The only downside to this is the x8/x8 lanes for XFire instead of x16/x16 but by the time the miniscule loss of performance matter you will likely be looking at a new computer anyway (hell, it will be a few years before you would even need to XFire 5870s)
Regarding the mobo, the x58 is pretty future proof so I think I'll stick too it, I really want this thing to last quite some time so I wont have to upgrade on for the next couple of years, except for adding another GPU to the mix
This is probably going to be the first and only time i spend this much money on a computer, so I want it to be extra good