Mr Batra :
i have a motherboard that supports 1066 mhz and planning to buy e8400 (1333mhz)
i want to know that upto what extend can i overclock it.
You'd need to let us know what board you have. A board that officially supports only 1066 MHz FSBs is probably going to be a Pentium D/65 nm Core 2 board and will not support the 45 nm E8400.
An E8400 on a good board should be able to get into the low 4 GHz range.