hey, same here. I just ordered my list of goodies. I went for 2x6950s (and still reserve the option to buy more later) instead of 6990s because the cost-reward factor was still to high. You can get 6950s in my area for 250$, but the 6990s are way up in the 700$-range.
What you need is a board that can host those graphics cards side-by-side. They usually take two slots, so the PCIe slots should be spaced further apart.
This board did it for me: http://www.gigabyte.com/products/product-page.aspx?pid=3880#ov
You can stick in 4 PCIe x16 graphics cards side-by-side. See the nice little picture in the promo.
Since most of the Bitcoin operations do use the GPU, I bought an AMD Phenom II x2 250, which was on sale. I also stuck in a 4GB DDR3 module.
What you need next is at least a 750W power supply that can power your 3 cards (or in my case later on 4). This one can do and was on sale at my retailer: http://www.ocztechnology.com/ocz-750w-fatal1ty-series-power-supply.html
For the case I recycled an old server case that formerly hosted an old Dual-Xeon server.
I hope it helps. If I were you I wouldn't throw my life savings into the 6690s, at the current bitcoin mining difficulty it'll take quite some time until you get your return on it, even with 2Ghash/sec.
Here are some numbers with $/Mhash figures and more ideas about rigs: https://en.bitcoin.it/wiki/Mining_rig