PCI slots are usually white, about half an inch in width, about four inches long, and almost as much in height. They don't have retaining clip at the front.
PCI Express slots are usually black or blue, narrower, and vary in length from an inch all the way to four inches, and they have a retaining clip at the front.
Here is an explanation of PCI slots, and
If your computer has only PCI slot(s), it will be difficult to obtain modern PCI graphics card, and it will be no better than your on-board graphics.