As we've seen, it's without doubt the densely foliaged forest scenes in Oblivion that really hammers system performance. Although reducing the grass draw distance makes by far the biggest difference to overall frame rates, disabling both the "shadows on grass" and "canopy shadows" options will also provide a big performance boost. But what you gain in playability you lose in pure visual drama. High frames or stunning visuals is your choice. Unless you have the very latest and most powerful 3D hardware, you simply can't have both with Oblivion.