EggXpert

I bought the a7da-s along with this memory

 http://www.newegg.com/Product/Product.aspx?Item=N82E16820227267

 However the MB runs the memory at 1.8v instead of 2.1v and I only get 400 instead of 800.

 How do I bump this up to 2.1v and get the advertised 800 out of the memory?

1.8V is the standard of all ddr2 memory. the 2.1V listed in the specs means that you could increase voltage to make it run in full speed if your mem is under 800MHz(if your motherboard allows you to increase voltage in BIOS). If your mem can reach the full speed at 1.8V, well-done. you don't have to increase voltage to mess it up. leave it alone.

ddr2 800 memory clock 200MHz I/O bus clock 400MHz 

It seems that there is nothing wrong with your mem.