What Oblivion has
HDR Lightning
> enable true white and a wider range of colors
> You need an HDR tv to use that (very expensive)

Good HDR Article found by BaffledMollusc

Physics
> allow rag dolling for dead bodies
> allow rag dolling for paralysis/telekinesis
> allow special effects on clothing & armors
> allow realistic interaction with objects
> allow the use of arrows to trigger traps or move objects
> allow realistic telekinesis
> 9000 types of objects are physics enabled
> Havoc is the physic engine used

Parallax
> an economical way to create the illusion of depth
> that's how the stones seems to stick out of the wall

Facegen
> a tool to generate faces using control points
> A convenient and easy to use tool
> You can customize your own look
> Modders can customize their npcs
> see the Race thread for more details on faces, hair and body
> included in construction set

FaceFX
Interesting info from Santera at TES.ag.ru
Gamasutra informs that OC3 Entertainment has released version 1.0 FaceFX a new product that lets game developers create expressive talking characters.
FaceFX replaces OC3 Entertainment?s Impersonator software, which has been used in over 30 game titles and was a Game Developer Magazine Frontline Award Finalist in 2004. Both products can be used to create realistic animation data using only an audio WAV file

At first look this info has no relation to TES, but we may find Bethesda >  TES 4: Oblivion in list of the customers.

Some info on OC3 Entertainment.
OC3 Entertainment provides facial animation technology to help create realistic talking characters.

Speedtree
> A level of Details system for tree
> Permit having a whole forest on screen without overloading the GPU
> Simulate tree/grass movement affected by the wind

Autonomous regions
> Isolate graphics details/clutter between regions
> see LOD section bellow
> permit Oblivion to be a separate plane of existence
> permit modders to create their own separate map
> permit a great amount of details for cities without rushing the GPU
> Autonomous regions are surrounded by invisible walls
> Autonomous region can share the same landscape, sky, etc...
> Of course they can also have different landscape, sky, etc...

Overall level of details strategy (LOD)
> permit to view longer distances/depth without overloading the GPU
> Walled cities are only loaded on a need basis
(avoiding the usual Morrowind hard disk crunch)


What Oblivion doesn't have
Displacement mapping
Only fairly recent cards support that feature. It is very demanding for the GPU.

Flex modifier on the body (for 3DStudio expert modders ;D)
What does it mean ?
> Hair will not fly in the wind
> Body parts won't bounce


What card should I get
My best recomendation is to wait until release date,
check the specs, if you fit the minimum,
buy the Game and check how it run.

A lot of features are often non essentials for a good game play.

Most importantly, simply droping resolution one notch will save you 50% of your GPU power.
That's 50% for each notch dropped !

cost to run higher resolution is proportional to number of pixels

1600x1200 = 1,920K pixels = +47% vs 1280x1200
1280x1024 = 1,310K pixels = +66% vs 1024x768
1024x768 = 786K pixels = +63% vs 800x600
800x600 = 480K pixels

The 6800 is only 60% more powerfull than an FX5600


There are however things you cannot negation
> You need Shader 2.0 or better
> You will neet to meet the minimum requirements

Also, VXSS (Dev) mentioned the computer he used to Develop Bloodmoon (released spring 2003) can run TES4 Oblivion (he use it when testing the game)

Upgrading before release is often a very bad move.
Unless your computer just broke.
You rarely need that upgrade for school or work :D