Is that a Dell Inspiron 545S ?
If yes, you need to order 2 X 2Gb 800MHz Dual Channel DDR2 SDRAM slots.
But to be on the safe side , since you want to continue to use the already existing DDR2 2Gb RAMs you have already installed, you need to be sure that you get the same make and model of the older 2GB RAMS so you don't see a performance drop,
Hope fully you're using a 64Bit edition of windows.....
To check the Make and Model of the RAMs that you already have and want to keep, you can either open the case and remove them and check the thing, but, preferred idea would be to download CPUz from http://www.cpuid.com/softwares/cpu-z.html
and check the Memory tab in it and then place your order for the new RAM