Choppy Video on all games Asus 4850

Make sure nothing is running in the background.
Turn off Cool and Quiet in the BIOS.
Run HD Tune and see how your drive is performing. (driver issues? have had this happen and made my SATA 2 drive transfer at like 3 mb/s instead of around 100 as it should)
Your processor is not one of the best for gaming, it is bottlenecking the video card somewhat, but I am not so sure that is the whole issue here. It should be enough to run these games decently.
Turn some of the settings/resolution down. Oblvian can still at times, especially with the processor you have, be too much for a 4850 if you turn everything to max.

The card was bought 2 to 3 months after the 4850's were released

Well, the original Asus card came with software (GamerOSD specifically) that would cause the game to detect the wrong graphics card and select the wrong shader. Also, look up tweakguides and go to Oblivion. He has really good suggestions for speeding that game up.

Start with the basics, most video/sound problems are driver related.
Set a Restore Point.
Download the latest drivers for: Motherboard, Soundcard (if any) and video card.
Download and install Driver Sweeper or Drivercleaner.
Uninstall the video drivers, drivers for the sound card (if any) and motherboard drivers in that order. After each uninstall restart into Safe Mode and run the driver cleaner program to clear out any components left behind by the uninstaller. WARNING these programs can erase registry entries, so check everything it is going to delete and deselect anything that looks wrong.
Finally restart into normal Windows and install the drivers: Motherboard, soundcard, video card, in that order, rebooting as required.