After my visit to the Apple Store today I have decided I am moving to the Mac platform. I am going to purchase the 27 inch i7 with the standard 4 Gig of memory but want to upgrade. My question is if I purchase another 2 X 4 Gig upgrade sticks to install myself will there be any issues with mixing the RAM? This option would be about the same price as having Apple install just an addition 4 Gig of memory.