I'll be getting a new computer in the not-so distant future.

I've never built one from scratch so I've been looking at a few sites that would build custom PC's, namely ncix.com and Ibuypower.com.
So I guess I'll just lay out the general components I'm looking at for sure.

For the Processor I want to use is the AMD Phenom II X4 965 3.4 Ghz.
The graphics card will be an ATI Radeon HD 5770.
For RAM I think 4gb should be sufficient to start, probably 2x2GB Corsair DDR3.
Will a 700w Power supply be sufficient? I know that the bare minimum for the Graphics card is 500w.
I don't know what to do about a case. I'd go pretty cheap on this, I don't need something that's decked out in super bright LED's, just something that fits all the components and looks half-decent.

Now what I really don't know what to look for is a motherboard. Any reccomendations on that?

Is there anything I'm missing?
Are there any components I listed that can be downgraded to save on money without a huge impact on performance?

It's going to be a gaming computer, so I'm mainly worrying about it playing games like Fallout 3 and New Vegas, Oblivion, maybe even Crysis?