iMac 27 inch 2009 has 4 gig ram. Upgraded to Maverick and computer SLOW, so going to add more ram. Which brand to buy? Crucial? was going to add 2 4gig sticks to make 12 total. can i still keep my original 4 apple ram and add the new ones or must i remove the original 4 gig ram. Also, I see that there are several different options for the DDR3 from each manufacturer. How do I know which one to get? The specs seem the same, but the model number differs. i was planning on buying Crucial 8GB Kit (4GBx2) DDR3 1066 MT/s (PC3-8500) CL7 SODIMM 204-Pin Notebook Memory Modules CT2CP51264BC1067 from amazon. there is a kingston for 170ish but not sure why more expensive and it is worth the price. thanks for help!