Higher power usage = higher heat generation by PSU = higher system noise level
I only see the first two correlate. The last one doesn't necessarily. Most PSUs now are bottom mounted and air either gets sucked in from the top or pulls air from the bottom, which doesn't affect the overall system temperature appreciably. Sure, the unit will get hot, but the chassis is tied down to a massive heat sink anyway: the case.
And a lot of PSUs out there that aren't hand picked by enthusiasts are pushed to that range anyway when working with a load. However, this is heavily dependent on a case-by-case basis. If your computer isn't doing anything most of the time, then the requirements can relax a little. If you're keeping your computer to do something constantly, then I would throw in something with a little more wattage to bring it to the 60% mark just so it's a little more efficient.
Besides, while yes, having a higher operating temperature affects a component's lifespan, it's even worse if you keep switching between high and low loads. But if we're going to play this game, we should recommend 1500W power supplies to every new build because the PSU is working very little.
Heck I threw in a 450W PSU in my latest build, I'm that confident given what I've measured in the past and my usage patterns that this is more than enough. The catch is that it's a very high quality PSU (a Seasonic 80+ gold model).ANYWAY
OP, since we got sidetracked here. If you're considering adding another video card down the road, this has to be a short term thing. i.e., you don't have the money now, but you will say half a year. The problem is that if you wait say a year from now, the next generation will be out and the next one after that should be due very shortly, which is similar to the position we are in now.
GTX 670's, the last generation card, is very hard to find now. And the ones that are going through retail channels are either asking MSRP (~$430) or they're not going to be any cheaper than around 75% MSRP. You could go with eBay, but I find that a little more risky. And if you wait until the next-next generation, chances are the card in this performance category will be almost double than what you get now, and may be going for the same price, if not a little more expensive.
There's also an adage regarding multi video card setups: next gen performance with last gen features.
In other words: either get SLI/Crossfire now or within a few months, or don't get it at all.