No, you need to uinstall the drivers first. Do the following in the order posted.
1 Uninstall ALL software associated with your current card; ie drivers and whatever else is installed.
2 Reboot and let windows install a standard vga driver.
3 Shutdown and pull the old card. Install the new card
4 Pw ON and install the latest driver from the ati site. Do not use the driver on the CD you received with the card. They are often many months out of date.
5 Once done with the driver install, reboot and have fun with the card.
A few other items you might want to think about;
1 The 4850 type cards use a lot more power than a mid range 8600. You need a quality power supply to power the card.
2 The ati card will be a lot bigger than the old card. Make sure you have enough room to install the card AND there is adequate airflow in the case to cool the card.