late 2013 iMac. OS Mavericks. I'm ready to order RAM and an ext SDD. I don't really care about booting from the SSD. But I want to install Aperture on it. Are RAM and SSD prices favorable right now, or better to wait. a) should I get a USB-3 dock or a Thunderbolt? b) does it improve performance (aside from Startup) to have the OS on an SSD. d) I am considering increasing RAM from 8Gb to 16Gb. If I wanted to go the extra step and end up with 20Gb, can I configure the memory with 3x4Gb chips and 1 8Gb, or must RAM cards be symmetrical.