High Quality - Low polygon assests

In your 2D textures within hypershade, use your normal maps as "Tangent Space Normals" instead of bump, it will keep most of your rendered high poly model details. 

Although there is a couple of problems with UDK it doesn't seem to like the normal maps very much (maybe because i didn't unwrap my UV's). Although it will look better with a correct texture as it keeps most of it's detail.

Rock brushes: http://www.turbosquid.com/FullPreview/Index.cfm/ID/451459

Pretty good techniques for rocks alone: http://vimeo.com/6332832