I'm looking at buying an iMac (second hand but very new), the specs are Apple 27" iMac SSD 32gb Ram GTX 780M i5 3.4ghz turbo to 3.8ghz but it seems to only have the 512GB Flash Storage installed. Can I add an HDD? If I can does that make it a Fusion system or is that a special configuration that can't be added aftermarket? I currently have an 2011 27" iMac with standard GPU and 8GB RAM. I assume I'd see a significant performance increase between the two (Bootcamp gaming and Graphic Design under OS X)? Thanks.