Can I add different kinds of RAM to my PC?

I have 2x2GB of G.skill ram in my PC right now with an i5 2400. Am I able to add in 2x4GB along with it Or would I just have to replace the 2x2GB ram outright. Current ram is this. http://pcpartpicker.com/part/gskill-memory-f312800cl9d4gbrl

Current motherboard is this. http://pcpartpicker.com/part/gigabyte-motherboard-gaz68md2h

Only reason im asking about this is that my memory usage is just about capping it and I want a little bit more headroom. memory Im looking at is http://www.newegg.com/Product/Product.aspx?Item=20-231-417&SortField=1&SummaryType=0&Pagesize=10&PurchaseMark=&SelectedRating=-1&VideoOnlyMark=False&VendorMark=&IsFeedbackTab=true&Keywords=%28keywords%29&Page=1#scrollFullInfo

or http://www.newegg.com/Product/Product.aspx?Item=N82E16820233186&nm_mc=EMC-IGNEFL120111&cm_mmc=EMC-IGNEFL120111-_-EMC-120111-Index-_-DesktopMemory-_-20233186-L03D

The g.skill is on sale for 10 dollars less though.