yes, DDR RAm is 184 pin stick, while the SDRAM stick is 168 pin, SDRAM stick has two notches while the DDR stick has only one that too located near the center so one cant get into the other slot.
256 MB isnt quite equal to 384 MB! if you are talking about performance, well, the amount of memory directly affects the virtual memory usage, which is the same in either case if the amount of memory is the same.
DDR memory is faster than the SDRAM so it can get faster results in memory intensive tasks, else its just the same.
<font color=red>Nothing is fool-proof. Fools are Ingenious!</font color=red>