Need PSU and Mobo tips (Finalizing Build)

I had a post about a week ago on the forums about a build I'm working on. That link is here: http://www.tomshardware.com/forum/369445-31-second-buil... . Since then, I've bought about half my parts and have changed my mind on a few things. I wanted to run my current build by everyone here at the forums, and I needed a few more tips. My current build is here: http://pcpartpicker.com/p/oeDM

PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: Intel Core i5-3570K 3.4GHz Quad-Core Processor ($219.99 @ Amazon)
Motherboard: ASRock Z77 Extreme4 ATX LGA1155 Motherboard ($124.99 @ Amazon)
Memory: G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-2133 Memory ($49.99 @ Newegg)
Storage: OCZ Agility 4 256GB 2.5" Solid State Disk ($174.99 @ Newegg)
Video Card: XFX Radeon HD 7870 2GB Video Card ($189.99 @ NCIX US)
Case: Rosewill CHALLENGER ATX Mid Tower Case ($39.99 @ Newegg)
Power Supply: XFX 550W 80 PLUS Bronze Certified ATX12V / EPS12V Power Supply ($53.99 @ NCIX US)
Optical Drive: Lite-On iHAS124-04 DVD/CD Writer ($15.98 @ Outlet PC)
Other: Intel Centrino Advanced-N 6205 ($33.99)
Total: $903.90
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2012-11-18 17:39 EST-0500)

I already bought the memory, on sale at Newegg for $35, the SSD on a sale from TigerDirect at $130, and the case and disk drive in a bundle on Newegg for $55 after shipping. I plan on getting the i5-3570k at below $200 in a sale, but if not I'll just suck it up.

Originally when I did this build I was going to go with a Radeon 7850 and eventually overclock to speeds close to the 7870, but I think I want to go with the 7870 instead. Is that price solid enough to buy now, or am I likely to get one a little bit cheaper in upcoming Black Friday and Cyber Monday sales?

I think my power supply should be sufficient, but I plan on adding a CPU cooler in the future and MAYBE Crossfiring another 7870. Will it be able to support both of these? I can always just get another PSU if I ever Crossfire them.

And lastly, is the Mobo I picked going to cover everything? I'm going to have to add an onboard PCI Wifi Card, wired is unfortunately not going to be an option. Just let me know if there are any other parts I should swap some of mine for, or if anything isn't going to work out. My original price point was at $800, I'd like to stick there and not go above $850.