My iMac has 8gb of ram from apple with 2 4gb chips. There are 4 slots so can I add 2 8gb chips from Crucial to add up to 24gb total or do I have to take out OEM memory before I can put Crucial in? Just got new iMac other day. My old early 2009 iMac only had two slots so mixing ram was not an option. I'm betting answer is I can't unless I buy same exact RAM from Apple.