As the title says.

You need a video card that can force AA, enable that in the cards settings then set oblivion to HDR.

Does an ATI Radeon 4850 1Gb have this feature?

If my x1900 XTX can do it, then your 4850 most certainly can. You should be able to use the Catalyst Control Center to force AA, and then set the game to run HDR.

Though I don't know if support for forcing AA in Oblivion comes with the newer Control Centers. I haven't ran both in a long time, but I do remember I was forced to use the Chuck Patch.

Ah, ok I'll try that then.

Just to add,

http://www.gamesas.com/bgsforums/index.php?showtopic=561101 if your Catalyst drivers are v6.10 or higher, it shouldn't need the Chuck Patch.

It worked! Thank you for the help.

I'll get a mod to close this.