Difference between onboard and AGP/PCI video cards
Onboard Video cards are not designed for games or video intensive programs (such as CAD). They are designed for people that wants a computer for general use. Onboard video has less memory than what you can get on AGP/PCI video cards. If you are going to use the computer for internet, word processing, financial or low end games, integrated/onboard video will work for you just fine.
If you are going to play high end games or use video intensive programs such as CAD, you should get a system with an AGP/PCI video card. They will usually have more memory and will allow you to use the high end games or programs. Because of this, the card will add to the cost of your system.
Some systems will come with an integrated video (meaning the video is onboard) and have an AGP or the newer PCI slot. This will allow you to use either the onboard video or add an AGP video card. When you install an AGP video card, it will disable the onboard video. On some systems you have to make sure the BIOS is set to use the AGP video.
If the motherboard has integrated/onboard video but no AGP or PCI slot, then your only option is to use a PCI video card (do not get this confused with the newer PCI cards). The PCI video card is slower than an AGP card but can offer more performance than an onboard/integrated video card. Some motherboards will not allow you to use a PCI video card and your stuck with using the onboard/integrated video.
The older version PCI video cards are slowly being phased out. You may not be able to buy a PCI video card unless you go to a computer show, EBay, or some store that carries older video cards. The newest video cards say they are PCI video cards but don't get them confused with regular PCI. It is a PCI video but it is the newer type of PCI. Those PCI video cards won't work on regular PCI slots.