New Build, Please Advise

Hi, I'm planning to build a new PC using some old parts and new parts. It will be used for everything I do, from video editing, medium gaming, school, work, and Photoshop. This is my second build. Components are as follows:

Case: Cooler Master Storm Sniper Black Edition (modded for 3x 200mm, 1x 140mm, and 4x 120mm case fans)

PSU: Split between 750/850HX and Seasonic X-750

CPU: Trusty old Athlon II X4 620 (planning to overclock to about 3.4Ghz)

Motherboard: ASUS M4A89GTD PRO SATA III/USB 3.0

RAM: G. Skill DDR3 1600 4GB(2x 2GB) Rip Jaws Class 9 1.5V

VGA: Either ATI 5770/5850/5870 or nVidia 260GTX/250 GTS (maybe light overclocks)

Cooler: Xigmatek Dark Knight (modded into push-pull config)

HDD: Western Digital Caviar Black 1TB 7200 RPM SATA 6.0 Gbps

ODD: ASUS DRW-24B1ST/BLK/ (DVD Writer 24x)

Monitor: LG 20" 1600 x 900 W2040T

I live in Los Angeles, so Newegg will charge me tax   but if any of the above items are shown on TigerDirect or other sites for a similar price, I will buy from them.

First of all, I need help deciding the PSU. The Seasonic X Series are efficient but pricey, and the Corsairs are a small step down but cheaper and carry a longer warranty. I have no qualms though about spending a few bucks for the X-750.

Next, will the PSU's listed be enough to Crossfire or SLI the 5770, 5850, and 260/250? If not, I'm fine with a single card layout.

Lastly, are there any other things that I have missed or should consider? Please post anything that should be added. Thanks.