How to run a laptop after replacing memory

Hello,
I have a Dell Laptop Inspiron 1520. 40 gb hard drive 1 gb ram xp os. Lately I have a hard time multitasking due
to warning that I'm having insufficient memory to run other programs and also when saving work done.
I decided to replace the memory to 4 gb (2 gb x 2). After I turn the power on there is only a black screen and the
power turns off automatically after 20 to 30 seconds. I repeated the same procedure and the results still the same. I decided to install the old memory (516 mb x 2) but still with the same results. It will turn the power on but it will not boot and the power turns off. Please help me.

Thanks.