to remove the old graphics card driver before you install the new one, but it isn't the end of the world if you don't.
Presumably your new driver came with its own setup routine? If that's the case I'm sure that everything is fine - have you experienced any problems since you changed the driver?
You can check the details of the installed driver version if you go to Control panel/system/hardware and click on the Device manager tab. You'll see your display adapter listed there, and if you double click on it you'll be able to read the driver version.