Mobo: Gigabyte GA-X58A-UD3R
RAM: 3x2 GB 1600 mhz CAS Latency 7
GPU: HD 5970 (or the two 6870s, if that's the only option)
HDD: Samsung Spinpoint F3 1 TB
PSU: A good 850W+ unit from Corsair, Antec, Silverstone, SeaSonic or XFX only
I choose the 5970 for a couple of reasons. First, it's performance is extremely good. It'll beat two 6870s. They're similar in performance to the 5850, and the 5970 is technically two 5870s on one card. The 470 isn't even a consideration, considering the massive power consumption and heat they'll produce in SLI. Also, the performance is similar to the 5970, but they're generally more expensive.
Second, a single 5970 will leave an upgrade path open. Instead of immediately going with a Crossfire setup, you have the option to drop a second 5970 in later. With the other two setups, you'd need to replace the cards, which would be insanely expensive.
Finally, the 5970 can be had for $470. The 6870s will cost $480 and the 470s will cost $490. They're all similar in performance at stock, but the 5970 leaves an upgrade path, uses less power, and leaves a lot of headroom for overclocking.
You should build this yourself. You get the best quality, best prices and more control. The custom builders are known for horrible quality, horrible prices and horrible service. It really doesn't take much to build it yourself, just a willingness to spend a little time reading up on the process and an hour or two to put everything together.