measure from the PCIe slot to the next impeding structure and then check against the size of the card you want. any decent website is going to list the length of the card in the specs.
if it truly is "PCI" as you stated and not "PCIe" then you may have an issue. there aren't many cards available anymore designed for PCI slots.
check Newegg.com and in your graphics card search check the PCI slot and uncheck AGP & PCIe. will return list of PCI only cards.