As is, before I start my own schpeel, a 650w Unit from a good vendor will do fine. It's here that you play with prices and constantly searching the egg for updates (wishlist rocks to watch prices...load it up) and can find 750W+ units for the same price the unit you're aiming at. I'm not suggesting you need more power, but and old 750w bronze refurb could be seen for $65 and actually be a "Silver" in your actual usage as I don't see you peaking
past 600. I don't want to go into it now, but going 100W past the peak usage is good, every bit after that allows the PSU to work less. It's not direct efficiency of the PSU, but the build as a whole. You can get 85% power efficiency in a build with an 80% PSU.
Good PSU's are PCP&C, Seasonic...and all builds those two make. I know corsair has a huge fan base...yeah...I won't go into it here.
My schpeel has weakened. If it's been 5-6 years, you're going to be shocked easily in a lot of your selections. An i3 w/HT will pull par with what I assume you're running, z77 may offer you way too much, Don't need a cooler unless you OC, Any SSD will improve you...and I'm going to say it: You could pull this off with iTX and build this in something smaller than a shoebox. However, you're a gamer and video card makers aren't playing that low-pro game (low pro is not "Single slot", but they're trying to say that. Low-pro is really half-height PCI slot). You need at least an mATX case and an mATX board.
You can trim a lot down on hardware to get a good gaming box then use the savings on peripherals. It doesn't have to be all on the box, so question everything around it as well. Does that Steel Pad and R.A.T. mouse look expensive? Chop $100 off the total build and purchase those vise a really kick ass CPU. Gaming isn't hardware as much...or even CLOSE as it is the Human Computer interface. Hardware very seldom makes a person better at games
. I say very seldom, but I would love to say "never" as I haven't seen it; but it could happen. The bottleneck in gaming is usually the human and/or the interaction to the box they built.
Don't blame the box. I'd play on my 5yo hardware and suck now, but it's only because I forgot the game