fix more ram, free up computer
the first part makes no difference, the game can and will only use 2GB of RAM, and certain OS dont allow for more than XGB of RAM, for instance, Vista 32-bit only allows 4 GB and Vista 64 only 8GB, though regardless its the amount of ram given to programs that prevents things from working faster...
as for the second, it could help, but only marginally...
unfortunately there is no real way to speed things up... though on smaller maps the time taken to unload is less than for larger maps