Help Me Choose Case/Mobo/Processors

What luck, I just recently built a fine computer for around $500.

First I took advantage of this combo deal that Newegg has going on:
Newegg.com - Computer Parts, PC Components, Laptop Computers, Digital Cameras and more!

Also, I really would not suggest getting that case. My last computer had construction like that (and cost just as much) and it was really ass to work with. Blood stains from sheet metal cuts do not a good casemod make. (However, if you are willing to deal with it, then by all means proceed.)
I decided to go with this case:
Newegg.com - Rosewill R5601-BK 0.8mm Japanese Cold Rolled Steel Screw-less Dual 120mm Fans ATX Mid Tower Computer Case - Computer Cases
and cannibalized the power supply from my old computer.
Since 500W should be enough for even a gaming rig I think the following power supply should suffice:
Antec Basiq BP500U 500-Watt ATX12V 2.01 Power Supply ANTEC INC BP500U
I also don't think the power supply included with that case you selected will be all that useful to you, considering it has only 1 sata power connection.

You will still need to get a graphics card but I'm sure you can use your head on that one.

As for information on motherboards. ATX, or standard ATX, has a total of 7 expansion slots. If you get one of those, you will need a case that can support up to ATX. The next step down in motherboards is mATX, or micro ATX, having 1 graphics slot and 2 expansion slots, which most cases that support full ATX will also support mATX. Then there is mini-itx which is really fucking small and is probably not worth looking at unless you plan on making your own custom computer case.

Also, the only reason these days that one would buy a multi-processor motherboard would be if they were setting up a server farm or performing massively parallel operations such as CFD or other simulations. Not to mention the CPUs that are compatible with such motherboards are not really optimized for standard consumer usage anyway(read:games).

Jam it back in, in the dark.

The primary issues I had with a case that of that construction was the poorly toleranced mechanical details, and the complete lack of deburred edges. I found that almost all of my hard drives would not line up properly with the screw holes that were provided, and so screwing them in and out ended up ruining the screws, making them one-time-use.
There is also the complete lack of broken edges, and just about every piece of sheet metal was a sharpened edge waiting to lacerate my hands.
There was a distinct lack of threading on the pci slot screw holes, leaving me to tap them myself. Some of these may be resolved, but I wouldn't hold out hope for that.
Finally, there was the issue that I had to remove some pci components or my graphics card if I wanted to switch out a hard drive, which gets tiresome really fast.

As for multiprocessor motherboards, I guess I was a bit uncertain what you were referring to. There are motherboards out there which let you mount more than one CPU module on them (example) and that was what I thought you were referring to.
Since most new processors out these days have multiple cores in them anyways, the motherboards that could go with them will have multi-core support, so thats a detail you really need not concern yourself with.

There's nowhere I can't reach.