YMMV: Alienware X51 Mini-ITX Gaming Desktop ax51-2146bk $500.99 Clearance, YMMV

You picked a Gt640, no no no, go for either a 7750 if you don't want to power the gpu or deal with the heat. Or preferably, grab a 7770(Nvidia just isn't cost competitive in the low price region. You will get much better performance for your dollar. I also say get a quad core, you'll be much happier, and if you have the cash, try and get a 128 gb ssd for around $70 sometime, it will be much better than the mechanical HDD. Here is my build for you

Same motherboard(where did you find it for that cheap!)
$89.99

Same DVD drive
$15.99

Intel Core i5-3450 Ivy Bridge 3.1GHz (3.5GHz Turbo) [newegg.com]
$194.99

COOLER MASTER Elite 120 Advanced Black [newegg.com]
$49.99(has a $10 rebate)

Antec EarthWatts Green EA-380D Green 380W [newegg.com]
$44.99
(this can easily power any GPU with a single pcie 6pin, or something beefier with adapter)

Seagate Barracuda ST31000524AS 1TB 7200 RPM 32MB Cache SATA 6.0Gb/s 3.5" Internal Hard Drive -Bare Drive [newegg.com]
$49.99

SAPPHIRE Vapor-X 100358VXL Radeon HD 7770 GHz [newegg.com]
$129.99 (has a $15 rebate)

Crucial Ballistix Sport 16GB [newegg.com]
$59.99

Windows 8 Pro 64bit [microsoft.com]
$40(or upgrade trick for $15)

Total my components = 529.94 (free shipping from newegg)
Your components(mobo dvd) = 105.98
Windows 8 = $40
Total = 675.92 -rebates($25) = 650.92

So definitely a better build then yours above, but maybe that is too pricey for you, we could cut a few corners by changing to get these items instead

SAPPHIRE Radeon HD 7750 1GB 128-bit GDDR5 PCI [newegg.com]
99.99 (with $10 rebate)
OR
Geforce 650 [newegg.com]
$99.99(no rebate available)

CORSAIR Builder Series CX430 430W ATX12V v2.3 80 PLUS BRONZE Certified Active PFC Power Supply [newegg.com]
44.99 (with$10 rebate)

Crucial Ballistix Sport 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1333 [newegg.com]
$29.99

This brings the new total to
620.76 - $30 to $20 rebates(Based off GPU) = 590.76 to 600.76

Still a superior build than yours.

You can only get Windows 8 for $15 if you have a good windows 7 product key that was activated recently(the key for windows 7 stays good). And you do it through this website [windowsupgradeoffer.com]. If not you are stuck with $40, which is still much cheaper than getting windows 7 and more future proof. Prices above reflect it at $40.

I do like your bitfenix case much better, but I was going for cheap quality components here, With any of these builds you have a great CPU with more than decent 1080P gaming GPU's. Also Either PSU can power a more beefier 7850 or Geforce 650 card, or if you use an adapter I personally know the Antec can support up to a Geforce 660 or a 7950(I have stressed that PSU a lot with bitmining). So the folks are right you can do much better, even with an itx build(but they are a PITA!)

Of course it is easier to go cheaper with a microatx build and less thermal work.

Thinking about thermal work, the case I listed comes with two fans, but the mobo only has one extra fan port so you'll need a splitter like this one [newegg.com] to get best airflow out of the box. You could always get a better fan as well, but these should be great until you decide you want to tinker with your build.