Citando Wikipedia:
Capacity
Per lane (each direction):
v1.x: 250 MB/s (2.5 GT/s) v2.x: 500 MB/s (5 GT/s) v3.0: 1 GB/s (8 GT/s) v4.0: 2 GB/s (16 GT/s)
16 lane slot (each direction):
v1.x: 4 GB/s (40 GT/s) v2.x: 8 GB/s (80 GT/s) v3.0: 16 GB/s (128 GT/s)