Are you willing to spend another $160 to buy another video card? Do you need the performance of 2 video cards? If not, don't go with SLi. 2 6600GTs wouldn't be worth it anyways.
Ok, do you know how to build a computer? Even if you don't, its easy to learn. Its not going to be anything like building blocks though, it IS a computer...but you could save $150-$200 rather than buying it.
Yea, the motherboard seems shady. Unless you want 2 video cards, don't get a SLi motherboard. If you're not overclocking, then there are some very cheap motherboards out there that support Athlon 64s.