The south bridge chip, also called ICH (I/O Controller Hub) is connected to the north bridge and is in charge basically of controlling I/O devices and on-board devices, like:
Hard disk drive ports (Parallel and Serial ATA ports)
USB ports
On-board audio (*)
On-board LAN (**)
PCI bus
PCI Express lanes (if available)
Real time clock (RTC)
CMOS memory
Legacy devices like interrupt controller and DMA controller
(*) If the south bridge has a built-in audio controller, it will need an external chip called codec (short for coder/decoder) to operate.
(**) If the south bridge has a built-in network controller, it will need an external chip called phy (short for physical) to operate.
The south bridge is also connected to two other chips available on the motherboard: the ROM chip, more known as BIOS, and the Super I/O chip, which is in charge of controlling legacy devices like serial ports, parallel port and floppy disk drive.
On Figure 4 you can see a diagram explaining the role of the south bridge in the computer.
click to enlarge Figure 4: South bridge.
As you can see, while south bridge can have some influence on hard disk drive performance, this component is not so critic to performance as the north bridge. Actually, south bridge has more to do with the features your motherboard will have than with performance. It is the south bridge that sets the number (and speed) of USB ports and the number and types (regular ATA or Serial ATA) of hard disk drive ports that your motherboard has, for example.
