What's about the best graphics card for about $300? And what's a PCI slot?
7850 probably. Before you go buying it though, please list your current spec (especially the motherboard model and PSU) because if you don't know what a PCI slot is then your setup may not be compatible.
As stated above, a radeon 7850 or a gtx 570 ,if u want to go the nvidia route, those are your best at or near the $300 mark, these cards and all modern graphics cards run on the PCI express bus (or pcie). Regular PCI is a ancient bus that is much, much, much slower, but can still be used for graphics just not really good for gaming graphics. If you know which mother board, or what computer you have post it and I can tell you if the system can run these video cards.
a GTX 580 would be way over the budget of $300, not to mention if he was gonna spend that much more money a GTX 670 would be by far the better choice.