BITS&BOBS, I thought as you did that Doom 3 would be more demanding on your brothers hardware than Far Cry.
Your brothers system is running on older version Catalyst drivers, they're currently up to version 4.8. You could try downloading these from click here and see if that improves the situation.
There's a new feature in these drivers and that's Temporal AA. click here for an explanation. This feature should help when playing Far Cry.
Hope this helps, dagwoood.