Advice/Comments for my New Build

I am building a systems for gaming. My goal is to build a system that will run, at a fairly high level, all current and upcoming games (say, Battlefield 3, Diablo 3). As well, as I am on a "budget" of sorts, I want to be able to upgrade the system going forward (i.e. add a second video card, etc.)

My questions regarding this build are:

1. I selected the power supply because it is SLI and Crossfire certified. I understand that this would allow me to add a second card and improve performance in the future - Is this true? Would you recommend another video card configuration?

2. How can I be sure that the selected power supply will work with my motherboard (if this is even an issue)?

3. Any other suggestions appreciated.

Thanks,

Motherboard:

ASUS P8P67 PRO, Intel P67 DDR 3 2400 ATX

CPU:

Intel Core i5 Processor i5-2500k 3.3ghz

RAM:

Corsair Vengeance 8GB

Power Supply:

Ultra X4 850 Watt, NVIDIA SLI & ATI Crossfire certifications

Video Card:

EVGA GeForce GTX 560 Ti FPB

HD

Western Digital 750GB, 7200 RPM, 64 MB Cache

CPU Heat Sink:

Zalman 9900 Max-R

Sound Card:

Creative Labs, Sound Blaster X-Fi Titanium

Case:

Antec 300 Gaming Case