|"sdram vs ddr ram"|
Technically, it's all SDRAM. The older 168-pin is SDR-SDRAM. The newer RAM is DDR-SDRAM, DDR2-SDRAM, DDR3-SDRAM, etc.
"my cmputer is 256 mb(168 pin) sd pc 100/133 supported.i want to increase my system up to 1GB of RAM.is it possible?"
If your system takes 168-pin SDR-SDRAM, that's what you'll have to use. DDR will not fit. As for how much RAM your motherboard will accept, we have no way of knowing because you didn't list the make/model. The link supplied by X86 *should* give you your answer.