There are a few strategies you can use to conserve memory for very large renders.

• Simplify your scene
      Simplify your materials. Reduce the resolution of bitmap textures for distant objects. Remove bump for distant objects. Reduce resolution of distant objects.

• Render in layers
      Select a few objects and use 'render selected objects' or 'render visible objects' and composite your image at the end with Photoshop or equivalent

• Render in tiles
      Select an area on your main camera viewport and render only that areas. Repeat for other part of the image, making sure you have overlap between tiles. And put together the fragments in Photoshop.

• agiel - www.linfa.net