No, don't use Legacy. However, you should review the sticky threads and others in the
Multi Booting forum section as there is good information there.
Since I don't know what you are going to use your system for (games, video rendering, audio recording or...), I can't make a recommendation. Do you want an ATX, mATX or ITX size motherboard? Lots of choices, but I need to know your use and environment before I make any recommendations.
However, I suggest you take a look at the CustoMac recommended system builds in the tonymacx86 Buyers Guide, the link of which is at the top of this web page, next to the Home menu item. The Buyer's Guide is very useful to start looking for components to build a system.
Note that we recommend you choose you memory from the the motherboard's Memory Support List (Gigabyte) or Qualified Vendor List (ASUS).