1) Buy 2 5870s and call it a day, eyefinity with more than 2 GPUs is total BS, just google for it and see! I have 1 5870 1GB and I play many (but not all) games smoothly with max settings @ 5992x1080 (5670x1080 after bezel correction). If you have the money, buy 2 of the 2GB versions of the 5870, the added vRAM will help in eyefinity resolutions. See the review on WSGF.
Another option is 2 Geforce 460 1GB in SLI and hooking up 3 monitors to them (2d vision surround), they're much cheaper than the 5870s, and offer more bang for the buck.
2) 6GB of system ram is more than enough. If you want to add more ram, make sure to buy the same kit and that they don't run faster than 1333mhz, as most mobos will have trouble running 6 ram dimms @> 1333mhz.
3) For the resulting system, get the corsair 850w PSU (it is enough), whether the TX model or the modular HX one, it doesn't matter.