The ATI 3850 video card is currently the best bang for the buck card out there right now. You should be able to get the 256mb version for around $170 shipped and the 512mb version for around $200 shipped.
Rebates for the video card are starting to pop up from various manufacturers. The reviews have been favorable and I personally am very pleased with my Diamond 3850 256mb version.
Remember that the card is PCI 2.0, but is backwards combatable with PCI 1.1. If you still have AGP and refuse to upgrade, they will be releasing a AGP 3850 soon. The card will support both AGP 4x and AGP 8x. Realize that your older CPU will likely be bottlenecking this card however.
Among the manufacturers making them are sapphire, diamond, visiontek, his, power color, gecube, gigabyte and asus. Most people are buying from sapphire and diamond and are very pleased with their cards.
As the card is newer, you may have some driver issues. The newest drivers did not work for me and I have to use the ones a couple updates back. Some people are also reporting issues with Vista. But who isn’t with any type of hardware?