Hi,
The simple answer is yes, if you get another HDD and build a RAID 0 Array, then you should see faster read/write speeds and an overall improvement in performance.
How much of a gain that you will actually see however, will be dependant on other factors such as how your PC is configured, if you are using other hard-drives and things like that.
The Southbridge on the motherboard, that controls the SATA3 ports, also shares bandwidth with the USB devices the SATA2 and IDE Channels as well as the PCI Bus, Firewire, Floppy and Audio, which does mean you could have a lot of items sharing bandwidth, thus reducing the overall efficiency of your system.