No fps hit with them at all or very small 1-5fps and like eveyone said a must download for oblivion.
From what I read oblivion has something like 800/600 res textures off at the distance beacuse of the limits on the xbox360 if they changed them on the pc people would complain that the pc is better or something like that.. So everyone gets the [email protected]
edit: I would just go to http://www.tessource.net/files/search.php
posted above and use search and put a check next to "More downloads then" and select 10,000 or 5,000
shows you the most popular downloads