What type? Whats it for?
DDR2 or DDR3?
DDR2 and DDR3 are not compatible with each other, the slot will only fit one or the other. So you would need to find out what you currently have.
http://www.cpuid.com/softwares/cpu-z.html
This software will tell you under the memory type the type you currently have installed. Otherwise, the make and model of your motherboard will tell us more!