I wouldn't be too sure about adding a second 7870 with only 550 watts. Remember, your suppose to try to not load your PSU much more than 80% load. 750 watts is the least I would use for a SLI or crossfire config, unless running something like dual 7750 cards.
A good quality unit (not even a
great quality one..!) will do 100% load 24/7 at it's rated intake temperature for the whole duration of it's warranty period. It's not that I'm
recommending to anyone to keep their PSU loaded down to it's max rating full-time, mind you. I'm just saying that 550W is plenty of power, much more so than is widely considered.
As for efficiency, it's a simple measure of expected actual consumption (i.e. expected impact on the monthly power bill), and to a degree, of the thermal contribution of the PSU. Efficiency is a ratio of AC input power (what goes into the PSU from the wall) over the DC output power (what actually comes out at the other end - what the PSU delivers to the PC components). So if I need 510W for my folding rig, and I have a PSU that is 85% efficient at that load level, I'll actually be pulling 600W from the wall outlet. 510W DC / 600W AC = 0.85 = 85% efficiency. The thermal contribution I mentioned is actually the difference between those two numbers, as the waste power has to be dissipated as heat. So the hypothetical PSU I used in this example will produce 90W of heat, about as much as a Sandy Bridge i7 at full load (stock clock, prime95 for example).
So, efficiency is desirable, as it serves both the interest of exploitation economy and better thermal performance (and thus possibly lower noise as well, due to a lesser need for aggressive cooling). But it is not a measure of a PSU's ability to deliver it's rated power, and it is not a direct measure of PSU quality, nor will it ever be.
At stock clocks, the OP's machine fits comfortably within a 300W power envelope at full load (meaning IBT/prime95/whatever + FurMark/Heaven/whatever + HDD random seek). This is my basis for claiming that he could even add another 7870 to the setup, not to mention a "measly" SDD + some LEDs that won't pull over 15W altogether in any scenario.