The best thing to do, especially when adding new hardware or switching to a different operating system is to clean out your old drivers before installing new ones.
XP and vista will have some default ati or nvidia drivers already installed, get rid of them, especially get rid of any dell drivers unless you have no other choice than to use them as they are always way outdated in my experience, getting the chipset drivers from nvidia was the right thing.
First thing though, use windows update to get your HD audio driver then reboot.
Then, download the latest ati 3870 drivers. (should be catalyst 8.7 at this time)
Download driver sweeper http://www.guru3d.com/category/driversweeper/
Install the program, uninstall your nvidia chipset drivers using add/remove, also uninstall any ati drivers that you installed, then boot into safe mode, disconnect from the net then run the program.
Be sure to check the nvidia chipset and ati boxes, it will wipe out your drivers, then reinstall in this order.
#1 reinstall the nvidia chipset drivers that you already downloaded.
#2 ati 3870 driver
The reason to uninstall then reinstall your chipset drivers is due to old default drivers still being on your system along with the new ones, this will sometimes cause issues, driver sweeper clears it all up.