enter the serial number of your i mac to check the memory type:
You also will want to double check with the vendor - before you spend money - have them check your work and explain their return policies if you happened to purchase an incorrect part or if the part doesn't work in your Mac.
It's much better to select a vendor that supports your needs - this includes finding the right product and support after the sale.
On Mountain Lion - you can use About This Mac from the Apple Menu and choose more information to get a memory summary as well as a link in the bottom right to your model's installation guide. That model-specific guide will let you know how many slots you have and whether you have to remove memory to free slots for additional chips or if you can just use a free slot to increase memory.