Rajan,
Allow me to clarify. RS-232 and IEEE 488.2 (GPIB) are two very different modes of communication.
RS-232 is also known as serial. Generally, it uses a 9-pin connector from the instrument to a COM port on a PC. It requires no extra hardware, but it is slow - only 1 bit at a time.
GPIB has a 25 pin connector and connects to a GPIB interface such as PCI, ENET, or USB. While it requires the purchase of a GPIB interface, it is much faster than serial communication - 1 byte at a time.
Most instruments can communicate through GPIB and serial. Which one you choose will be based on speed requirements and willingness to purchase additional hardware.
For additional support resources: http://ni.com/support/gpibsupp.htm and http://ni.com/support/ser
lsupp.htm
Good luck with your application.