Amusingly enough it is possible to still get enough memory that Windows 7 Ultimate won’t cut it. Kingston makes 32GB sticks and some Intel boards have eight slots for 256GB of RAM. Microsoft must have thought 512GB was a very decent upper limit but we might hit it before too long. 
http://www.newegg.com/Product/Product.aspx?Item=N82E16820242003 - Kingston DDR3-1866 32GB