OP, I'm confused, are you going SLI (obviously not for LOTRO, for other games) or not?
Your original Corsair 750W is more than enough for any video card. One editor at Maximum PC has been powering his Geofrce Titan with a 750W name-brand power supply and it's been fine for a long time.
If you're going SLI with 2 or more video cards, bumping up the 850W would not hurt. Purely just me, I'd invest in a single new Ti 780 before dealing with the hassle of SLI, especially if you like games like LOTRO where SLI will not help and probably hurt.
IMO get Windows 8.1. I support computers for a living and play pc games a lot, and Windows 8.1 from a hardware&software technical perspective is going to be light years superior to Windows 7. Whatever OS you choose, double check that your motherboard has drivers for it.
Overall, looks like a good build, hope you have a great time putting it together and playing on it.
[COLOR=Lime][SIZE=1]other favorite middle-earth related games: The One Ring RPG, published by Cubicle 7; LotR: The Card Game, published by FFG[/SIZE][/COLOR]