These tweaks are straight from a Oblivion developer:
http://www.shacknews.com/ja.zz?id=11842902
DISCLAIMER: Use at your own risk. Back up your Oblivion.ini first. Please do not request official troubleshooting support with any changes to your .ini file.
Here are some tweaks I play with:
uGridDistantTreeRange=15
Raise to increase area in which LOD tree billboards are drawn - looks quite nice doubled if your comp can take it.
uGridDistantCount=25
Raise to increase the number of LOD tree billboards drawn - looks quite nice doubled if your comp can take it.
uGridsToLoad=5
Raise to increase the X/Y dimension of the number of high detail exterior cells loaded - should be an odd number, 3 or greater. This is quite a performance hit with anything above 5. If there are 25 cells loaded at a time with a value of 5, then there are 49 cells loaded with a value of 7 and so on. I would not recommend going above 7 or 9, but terrain looks nicer if your comp can take it.
bAllowScreenShot=0
Set to 1 to allow the PrintScreen key to take screenshots.
bFull Screen=1
0 = windowed, 1 = fullscreen
iSize W=640
iSize H=480
Current screen resolution.
bUseWaterReflectionsMisc=0
bUseWaterReflectionsStatics=0
bUseWaterReflectionsTrees=0
bUseWaterReflectionsActors=0
Set these to 1 to allow these kinds of objects to reflect in water.
fDlgFocus=2.1000
Raise to pull camera back some in dialogue - I use a value of 4 or so.
bForceFullLOD=0
A value of 1 forces foreground trees to always render at the highest detail - I haven't found this to be a huge performance hit for the visual benefit, your mileage may very.
iMaxGrassTypesPerTexure=2
Raise to allow more unique grass types per terrain texture - 4 should be enough to let you see everything possible.
Haven't purchased the game myself yet, so I can't test them..

If anyone could post some comparisons before and after the tweaks that would be greatly appreciated..