The throughput on USB 2.0 is 480Mbps. That is both ways so you can achieve about 240Mbps writing/reading. If you want to take full advantage of RAID be sure to use SATA 3Gbps or SCSI.
What happens is lets say for example you send a 480MB file to the RAID0 array through USB it is cut in half (at least) and the rest is cached so that more data can be sent; the USB Hub processes the data to the HDD's small pieces at a time (<=240Mbps) whereas SATA can process it at <= HALF of SATA 3.0Gbps for example.
This is one reason why you see External HDD's sold with 5400RPM and not 7200RPM (because it's overkill; you will never use the additional 1800RPM because USB is not fast enough yet).
To let someone who may not know. When I say half I am referring to broadband communication. This is because all (modern) media takes into account reading and writing. So you can read at most half the speed of the said speed. Another example is 802.11g 54Mbps TOTAL read AND write... Cut that in half because it's a broadband media and you can at most read and/or write at 27Mbps.