Grass is a hard one, Here's a place which has some grass models http://www.3dplants.com/ They work good for patches, but you can't really fill up the whole scene with them. Another option is to use a flatten cube or even a 2D plane with a decent tex & trans map for distant grass ( doesn't work to good close up ) Infinite planes don't work to good for this, because the tex map gets to stretched out. Then there's always the mask render and post work.

Stephen Ray