up to what type of spec can your motherboard handle?
also in order to recommend a card its first good to know the type/spec of the system it will be required to run on.
Im not a graphic card expert but i have fitted one or two in my own pc's at one time or another but it would help others who do have the expertise to help you
In order to disable the onboard graphics that you already have you will probobly have to go into the PC's BIOS settings.
PC's BIOS or CMOS. Most computers will give you a short startup screen when booting up that states something such as: "Press Del for setup or other possible keystrokes may include F1, F2, F10, F11, F12, Esc, Alt+Q and Alt+S. Once in setup, you may follow the menu prompts to navigate your PC's BIOS settings for anything that may disable Onboard Video.
Have a look and see if this is correct on your PC and make sure that you buy a card that is able to run at the speeds accepted by your motherboard, if you dont have a manual you will prob have to goto the manufactures website and check your AGP slot's speed in there before making any changes to your system.
Im sorry I cannot be of more help - I dont tend to mess around with the BIOS unless I have to but I have had to use it before now obviously to set CPU/memory/graphics cards setting on upgrades I have done myself at home.
If in doubt pay someone who 'knows about these things' to do it for you, lol - good luck!!