If you need a monitor and whatnot too, that basically makes your budget for everything else about $850 +/- .
You cannot build a high end gaming PC for $850 unless you steal half the components. You can make a decent low-midrange PC, but there are going to be compromises. Something along the lines of this: http://www.newegg.com/Product/ComboBundleDetails.aspx?ItemList=Combo.770956
is going to be your best bet, and even with that you are looking at anohter $100-$200for the operating system. That particular platform has a few problems (the motherboard is sh*t, the case is sh*t, and the power supply is too low a wattage for a gaming PC) but it has the advantage of being cheap. You would realistically be looking at upgrading at least the case, power supply, motherboard if you ever wanted to try some sort of SLI gaming setup. With hard drive prices being what they are at the moment, you aren't going to be able to go much cheaper than something along those lines unless you drop the processor down to an i3 and drop the video card down to the cheapest piece of junk you can find, which would bring it in at around $600, but wouldn't be capable of really even loading FFXIV.
You might be better off going with a Dell or some other premade type computer that comes with a monitor. They aren't going to be very upgradeable, but they can generally do the low end PC's cheaper than the build your own route at that level, and that gives you a starting point for building a higher end one later. Something along these lines maybe: http://www.dell.com/us/p/xps-8300/pd.aspx?c=us&~ck=mn
That or http://www.ibuypower.com/Store/Intel_Z68_Core_i5_i7_Configurator