different types are
x1
x4
x8
x16
2.0
x1 is mainly for a small variety of sound/gfx/NIC cards, not often used
x4 will bottleneck high-end cards, not sure about low-end
x8 runs most cards
x16 ideal for most gaming cards, standard
2.0 is for future cards, x16 cards work in them and ATM all 2.0 cards work at full potential at x16