On some older computers using old style PCI video card might be faster than using AGPx1 or x2 style slot. That is because they make some pretty decent PCI video cards still, but don't make any AGP x1 x2 cards.
Plus as has been said already the newer AGP cards might not work in those slots.

PCI-Express x16 I think is what you need for the new PCI-E video cards.