I recommend to do 'fake' FX. Use sprite (with SpriteViewMode=4) or "X" mesh with two planes (like 'Laser' in cosmic games) + simple sun-beam texture. And use BlendMode=3 for rays with small alpha value. If you good modeller, that 'fake' FX will beautifully.
But god rays are different, they are dynamic and they even appear on foliage and trees. Yeah it seems I'm asking for too much :P Thanks man.