Assistance with New Build - Gaming (and light work)

Instead of the Corsair Obsidian, I'd go with a Bitfenix Ghost. It has the same sleek styling, it's equally well built, it's slightly cheaper, and best of all, is completely soundproofed.

For your graphics card, the 680 is a waste of money. It's only 5% more powerful than a 670, but it costs 25-30% more. Save yourself the money and get basically equal performance. (It is equal when you consider that spending $100 for 5 more frames a second doesn't get you anything when you're already maxing games at more than 60fps.)

For the hard drive, don't get an RE4 - that's an enterprise drive designed for RAID setups, and doesn't have a number of the features you want. Go with a western digital blue, which will be cheaper and faster.

You don't need a 750w power supply by any means - you're fine with a 550w for that rig, unless you plan on using SLI in the future.

Finally, as for the everything else, it's good, but I highly, HIGHLY suggest you get a 128GB ssd. It'll make more difference than any other upgrade when it comes to making your computer feel fast and responsive.

Then, just a few points of interest for the peripherals:
1) Consider getting a cheaper monitor and saving up for a BenQ XL2420T. It's the best of the best, and a 670 or 680 will max most games at 120fps without breaking a sweat. Playing a multiplayer shooter on a 120Hz monitor with the features that one has just feels like cheating.

2) The mouse is a personal choice. I personally dream of the old Intellimouse Explorer 3.0, which was as close to a perfect mouse as I've ever found. I've tried a lot of others since then, and the only one that comes close to my liking, is a razer deathadder black. I was reluctant to try it, because usually razer makes cheap plastic she-ite, but the black edition deathadder is actually very well built, and stays true to the form of the intellimouse explorer. (But again, it's all personal - the logitech G9x, one of the most popular gaming mice out there, makes my hands cramp after just a couple minutes using it.)

3) As for the keyboard, and this is important, buy a $20 normal keyboard, and save up for a mechanical keyboard. If you spend money on a "gaming" keyboard and it doesn't have mechanical switches, then you're paying $50 extra for lights under the keys that you're never going to look at anyways.

Here's some more... literature... on mechanical keyboards and why they're desirable. http://www.pcworld.com/article/240939/mechanical_keyboards_should_you_switch_.html

(When I switched, my WPM went from 80 words a minute to 108 within five minutes of adjusting to not typing on an ergonomic keyboard anymore.)

Glad I could help! Yes, 8GB of RAM is MORE than enough - it will simultaneously run battlefield 3, photoshop, AND 30 tabs in chrome.