Okay so you want to start out with an ATX case with six 120mm fans, to cool your dual GTX690's in SLI.
No seriously, go over to HardForum and just tell them your budget and use, or look at the dozens of other similar threads and look into those components. Get an i3 or i5 Intel processor with integrated graphics, a base ATX motherboard, 6 or 8 GB RAM. A nice upgradable case is only about $100.
$400 will actually build a somewhat decent computer if you find the right prices.