You can change the texture for the eco on the general tab. You can also make it procedural by double clicking the terrain & using the drop down to switch over, you will have to re-populate the eco afterwards though. My machine is 266ghz, 1gb ram & I just posted a huge procedural with large ecosytem with global illumination. 1024 size render in 3hrs or so. The first attempt was 37hrs & 55% so I went back & changed the GI quality to -1, this then rendered in the 3hrs. Hope that helps.