New PC Build

PSU = Enough
MSI = Not much people recommend it. Asrock and Gigabytes are the favorites here.
670 = EVGA is the best with warranty.

To start of answering your first question: Yes, but you may need a new one IF you plan to SLI the card with another 670. Most people won't do this as it is WAY more than enough power.
Your second question: the MSI board are "fine" for overclocking, make sure you know what you are doing. If you dont know what you are doing: look into the Asrock z77 Extreme4 or the Asus z77 V-(X) for overclocking.
To answer your third: I really do not recommend EVGA they run hot, meaning there is little room for overclock. In general a lighting edition MSI card would end up being a better investment as you will get more power out of the card than you would the EVGA card. Also stock speeds on the MSI card are faster than that of EVGA.

Hope this clears things up a bit. Feel free to ask further questions.

I'm not looking to overclock the graphics card at all. Is there really a big performance increase if you do? I'm only looking to overclock the CPU, but I've never done it before so I'll look at the Asrock and Asus boards. Is OC'ing relatively easy?

Overclocking is simple as long as you look up a proper guide for your specific motherboard and CPU. You have to know the limits of the board and the CPU and then you will be perfectly fine. It can be a very... Interesting process and can take a few hours. But yeah there are plenty of guides on how to overclock just make sure you have the proper guide.

As for the GPU there really is not a large increase in speed between the 2 at stock speeds so you can stick with the EVGA card.

Yes. I wouldn't OC the GPU.

Ok, thanks.

Here is what I'd recommend... I know everyone has their own opinion but this would do everything you want and more, plus cheaper!

CPU​
Intel Core i5-3570K Quad-Core Processor
http://tinyurl.com/ag9da7q
​$223.79 + FREE Shipping = $223.79

CPU Cooler
​Cooler Master Hyper 212
http://tinyurl.com/aktuhwh
$25.98 + FREE Shipping = $25.98

Motherboard
ASRock Z77 Pro3 ATX LGA1155 Motherboard
http://tinyurl.com/a2pwqj9
​​$89.99 + FREE Shipping = $89.99
​
Memory
G.SKILL Ripjaws X Series 16GB (2 x 8GB) DDR3 1600
http://tinyurl.com/bxy7vpg
$89.99 + FREE Shipping = $89.99
​
Storage
Crucial m4 128GB 2.5-Inch Solid State Drive
http://tinyurl.com/ay6cn7w
$116.55 + FREE Shipping = $116.55
Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive
http://tinyurl.com/bawe84c
$74.99 + FREE Shipping = $74.99
​
Video Card
PowerColor Radeon HD 7950
http://tinyurl.com/b8dksvc
$294.99 + FREE Shipping - $20 Rebate = $274.99
​
Case
CM Storm Enforcer
http://tinyurl.com/bzxk8ao
$79.99 + FREE Shipping = $79.99

Power Supply
Raidmax 850W 80 Plus Gold
http://tinyurl.com/bkbhosc
​$93.49 + FREE Shipping = $93.49

Optical Drive
Lite-On iHAS124-04 DVD/CD Writer
http://tinyurl.com/b7hm9l4
$17.99 + FREE Shipping = $17.99
​
Operating System Software
Windows 8 System Builder DVD 64-Bit
http://tinyurl.com/byy8ja9
​$95.88 + FREE Shipping = $95.88

Base Total: $1,203.63
Rebates: -$20.00
Shipping: $FREE
Total: $1,183.63