Ok, I'm by no means an expert, but I've slapped together 2 rigs and here's my two cents:

GPU - Let's skip this till the end. Lots of opinions here worth considering.

Motherboard - Not sure why you are buying 2. But there are cheaper and/or better options on Newegg. This one will allow for 5 cards rather than only 4 and costs the same after shipping. This one is the same as your original, but with cheaper shipping. I assume you're going AM3+ because you already have cpus (since you didn't mention any in your post).

Riser - There's no need for 16x-to-16x risers. 1x-to-16x will operate fine in a 16x slot, and they're far easier to manipulate. Quality is highly variable with these, which is why I am a big supporter of this type or this one if you want a longer cable. I got mine from a seller on ebay for about $20 a pop and am using 6 of them. All are powered so never a concern for whether your card is being supplied with enough juice.

USB Drive - Sure, looks fine, but you don't need bigger than a 4 gb (8 gb to play it safe), so cut your costs and get something smaller.

RAM - cgminer runs best with 4 or more gb of RAM. But ram sticks each consume a few watts of power (though the actual size of the RAM stick does not increase the power consumption. The speed of the RAM does affect power). So for the best power optimization, use a single 4 gb stick on your boards.

PSU - It says "more than 80% efficiency" but has no 80 Plus rating (silver, gold, platinum). This is extremely important because if your PSU fails it will, at best, put you out for a bit until a replacement is installed or, at worst, damage some components. You don't want to skimp on your PSU, and you want to make sure you have sufficient overhead to allow for the max TDP of all of your components simultaneously. I am personally running 4x R9 270s on an AX860i (which is platinum rated), so I can vouch for its ability to power everything (including 4x powered risers). But Corsair charges a ridiculous premium for their AX line right now, so find an 850w or higher PSU with a gold rating and you should be fine (and a good warranty never hurts). This one is Gold rated and modular to boot! Modular definitely helps when you're trying to weave your way through the tangled mess of an open-air rig. If you're going to run more than 4 cards on the mobo, this one gives you a gold rating on a 1000w psu.

Alright, now back to the R9 270s... I personally have 4 of these, but they only cost $209 when I bought them a couple weeks ago. The prices have spiked recently, and you might be well served to consider waiting for them to fall back down. Nvidia just released the 750 ti which supposedly hashes at 300 kh/s with only a 60w draw on power, and they're going for $150 a piece, so they ought to create some solid competition for the R9 270s since you get more bang for your buck from a pair of 750 ti over an R9 270. But, obviously you get a higher hash rate from maxing out the capacity of a mobo with R9 270s. In any event, the price of the 270s will probably come back down as entry-level miners opt for the more affordable (and more accessible) 750 ti.

I'm getting 475 kh/s out of my 270s, which is about par for the course. I've heard a number of horror stories surrounding XFX and Powercolor cards, but if the cards were all faulty, the company would be out of business. So your mileage may vary with those brands. MSI TF 2 line of cards is supposedly the end-all-be-all, but they're nearly impossible to find. Asus is always a quality product, but there's a lot of talk about the cooling-solution being insufficient for long-term, 24/7 mining.

So, that's my 2 cents. Hope it helps.