Nothing wrong with it. PSUs have an ideal % band of operation whereby they deliver best efficiency. This differs, but all this really means is that by operating outside this band, your PSU may be pulling a bit more from the wall than it would if you were in this band. 60-80% is what i have read for most PSUs, so you will be more like 20%, but there is no harm in this.
To expand on what he said, 80+ tests PSUs at 20%, 50%, and 100% capacity, and efficiency must be high for certification. Below 20% load though, efficiency drops off, sometimes very quickly. On a sub-100W load (10% or less), that PSU may only be 70% efficient, or even less. Any time you're not playing games, that's where you'll be. With a 500W PSU, that's 20% of its label, so it will be in a more efficient part of its range. Unless you're planning to run three or four high-end graphics cards, you will never need 1000W; if the 700W PSU is a decent one, it will support two of almost any graphics card; just use that.
What brand/model is your current 700 watt? Do you just feel like buying somethng and came across a good deal? I don't understand why you would do ths. Buy things that make sense, and buy them when it makes sense to buy them. There will ALWAYS be SOMETHING on sale.
There is no risk using the 1000w with a Asus M4A87TD EVO mobo is there? I don't think there will be. I completely agree I do not need that much power but since just as cheap if not cheaper than a equivalent modular 700-750w PSU I figured, why not!