Have you patched it up to latest build? You should be getting much more out of it than that. I know there is also an issue with quad core and this game, try starting it up then alt tab out when on title screen and open task manager, then in processes find the Oblivion exe file and right click, then select processor affinity, leave two CPU's with the tick in the box and untick the others then ok it. Then alt tab back into the game and see what you get when you start.
Then theres the Tweakguide which you already mentioned :
http://www.tweakguides.com/Oblivion_1.html
I would try the affinity first and foremost though.