Quote from Forbin :

PCI-E 3.0 16x: 16GB/s (current cards are probably only using about a quarter to half that)

more like an eighths or less
http://www.hardocp.com/article ... idth_perf_x16x16_vs_x4x4/

Quote :

PCI-E 2.0 1x: 500MB/s (i.e. 0.5GB/s) (this is that little tiny slot)

Granted, there are some cheap video cards that run on PCIE 1x.

judging by that x4/x4 test i wouldnt be surprised if the hit from going down to x2 or even x1 is barely perceptible
the real problem with usb will be that iirc its latency is rather shit