The very BEST way to allocate more memory to Bryce is to go buy more memory. I have a scene that is only slightly above 150 MB, file size, but it will not render until I get 1 GB of RAM. I've had scenes that were larger and rendered them fine with 512, but this particular scene has many, many .obj imports and .3ds imports, and crashes if it's opened for more than a few minutes.... If you're using Windows, you can change the Virtual Memory settings to get a little bit of headroom. I set mine to a minimum of 512 MB, and a Maximum of 1 GB, which seems to be enough for Bryce, usually...