It will, but it is different.
The PCI bus offers 133 megabytes per second, where USB 2.0 offers 480 megabits second. The difference being there are 8 bits in a byte, meaning USB 2.0 really gives about 60MB/sec transfer.
So no, you will not be limited. Mbps (megabits) is just a marketing gimmick for people who aren't able to spot the difference between Mbps and MB/sec.
Also, firewire is 400Mbps.