Unfortunately not. As the game is 32-bit, it can only address a maximum of 2GB no matter what (the balance of whatever it can get from main memory after Windows and other programs take their share comes from virtual memory). You can save a bit of memory by reducing the planet surface variety slider in the options, but there's not a whole lot that can be done in DA beyond that. TA has a number of mechanical/feature changes (eliminating the old planet textures system almost entirely, etc) that reduce it's memory footprint so that big maps are less likely to run into the limit.

Most people play TA, which has the aforementioned changes to reduce memory usage (which were enough so that a new map size could be added beyond gigantic, even). There's not really a solution for DA, aside from cutting the planet textures back and playing with fewer planets.