X1950 Pro, better performance and image quality.
And unless you plan on keeping the X1950 Pro for a good while there's no reason to pay the extra for the 512MB version.
This one if it comes back in stock when you order:
http://www.newegg.com/Product/Produc...82E16814102070
This one otherwise:
http://www.newegg.com/Product/Produc...82E16814102061
*The first one has a much better heatsink (it retails for about $40 on its own) and has the highest stock memory clocks that I know of for X1950 Pros.