USB 2.0 vs USB 3.0 (Memory Card Reader-30Mg/s vs 45Mb/s) (Sony DSC-150)

Greetings,
I recently purchased a compact digital camera (Sony). The camera accepts a minimum class 4 memory card. My computer is a Hp P6370t, in which 2.0 USB ports are included.
The theoretical maximum data rate in USB 2.0 is 480 Mbit/s (60 MB/s) per controller and is shared amongst all attached devices. Some chipset manufacturers overcome this bottleneck by providing multiple USB 2.0 controllers within the
southbridge.
This said, I believe that Hp incorporates Southbridge, containing three controllers. Would this mean that the "maximum" read speed would only be about 20MB/s? I'm trying to determine, 'without testing', whether a memory card rated at 30MB/s would differ than one rated at 45Mb/s. Naturally speaking of the same size memory card, (16Gb/s).
Any and all replies are very welcome. Thank you very much.