Who made your system and do you know the model number? I should be able to find out that way.
If not do you know the exact make and speed of the CPU in your system.
Now is a great time to buy RAM - it is at near record low prices. I was able to buy 8GBs of ECC RAM for less than 100.00 - normal desktop ram is about 30% less.
What is a good source for low cost memory?