You're still missing it. You want a PCIe x16 video card (PCI Express). The PCIe slot is specifically designed for a graphics card. Any PCIe card will run circles around its PCI relative.
The brand will be decided based on your needs. On the NVIDIA side of the house I wouldn't buy anything above a 600 series of a card. For example: 7600, 8600, or 9600. Just make sure to share your findings here so that we can take a look at them. While customer reviews may rave the card is exceptional, we might notice that the card has abnormalities or isn't that great of a deal.