I started with a 550 athelon 128 ram, went to a 750/256 pc 133 and now on a 1.4/512ddr. Bottom line, Bryce uses processor so its not a matter of how many instances of Bryce you have open, but a matter of how much CPU you have available for the render. If you are rendering in one of the open Bryce programs, then I would recommend you not use the others for anything but working. If you need to do more, do the bryce lightening and render over a network.