in the categorizes there is mining and plotting threads that people often speak about how long it takes to plot, which software to use etc. heres a link for a pc usb3 hub http://www.ebay.com.au/itm/ORICO-7-Ports-USB-3-0-PCI-Express-Expansion-Card-Adapter-15Pin-SATA-Power-Supply-/262752290152?hash=item3d2d41c568:g:Rv4AAOSwdGFY0YCF
you could buy that model there are many. The 4 port models usually link 2 ports together i believe many motherboards do too. You will know when you reach a bottleneck when your miner read time gets beyond say 40 seconds. If it goes over 60 seconds per round you will loose submissions.
The PC you've mentioned the one with an i7 processor should make plots nice and fast with xplotter