The PC may be in the neighborhood of 400W and the LCDs might add another 100-150W. At ~550W peak load if you plug everything on it, 800VA might cut it pretty close. (The real-power rating of UPSes is 60-70% of their VA rating)
You would have to upgrade to 1kVA if you want some safety headroom.
Either way I think the 800 would be fine, but I suppose that would mean it would be less of a bad idea to lean towards a higher amount.
You are right the FX doesn't hog much power but the rest of the system does. I have few hard drives H80 closed water cooling full system fan setup which means 4 very large fans + couple of small once. Aside from that i have bunch of LED lights in there and for now a single GTX 680 which is OC'ed so that hogs a'bit of power. Not to mention the 2 LCD's gaming keyboards + mouses and i plan to probably add a second 680. + a severely overclocked FX 8150
PS: ( Assuming of course my FX 8150 doesn't bottle neck the second 680 )