First off, if you are buying an ATI card, do not get a SLI board. ASUS just released their newest Crossfire board that would be excellent for you and very stable to boot. It is called the A8R32-MVP and it is the best upgrade path with an X1900. Here is a review:
http://www.anandtech.com/mb/showdoc.aspx?i=2697&p=1
This board is also supposed to be an unreal overclocker. Your best choice for a board.
Of course you should get the 4400+ if your budget allows it, but the 4200+ is also a very good choice.