Lets saaay, i download the drivers my GPU comes with, and i go to ati.com to download the latest ones for it, will the ones i download replace my older ones? Or will i just have to delete the outdated ones. (Hehe i know nothing of drivers)
Before updating drivers, I suggest setting a restore point to go back to if you have problems with the new driver. Also, it doesn't hurt to store a copy of the old driver, unless you will have easy access from the GPU install disk.
You should be familiar with the feature in Device Manager that lets you roll back to the last driver. In Device Manager, click on the hardware - in this case the Display Adapters - to show the particular device, right click, and select properties. Select the driver tab and you will see a button to roll back driver. Of course, this is only useful as long as the new bad driver is not so bad that it prevents you from going through this process.