Generally, drivers may be updated by downloading them to a folder on your machine per the vendor's instructions, opening Device Manager, locating and highlighting or right-clicking the device to receive the updated drivers, selecting "Properties", selecting the "Driver" tab, clicking "Update Driver", and pointing the Update Driver Wizard to the folder containing the new drivers. No uninstallation of the existing drivers should be necessary, and the system should retain the ability to "Roll Back" to the previous driver version if the new one proves unsuitable.
The Catalyst 5.12 drivers are the latest ATI-supported XP drivers for the ATI 9700 series cards. Dell's Drivers Download page offers relatively recent (as late as September '05) driver updates for the Dell-Branded OEM version
of the 9700-series card, for various specific Dell machines.
Neither ATI nor Dell recommend use of the Catalyst drivers with the Dell-branded version of the card, but if it works for you, it works for you; reports have been mixed, so far as I've seen. Your call if you have the Dell-branded card - prolly be a good idea to make sure you've got a backup of your currently installed Dell-proprietary drivers before installing the Catalyst drivers directly from ATI.