My PC is an overclocked gaming/DAW PC and one thing that was recommended to me was to always buy your RAM in matched sets. Mine is 3 years old now and I can't even get extra RAM with the same CAS timings as my existing RAM and when I was looking around on forums to see if mixing the CAS timings was ok, the answer was no - even if it's the same clock speed, make and model of RAM. Mixing RAM and in particular doing it on an overclocked machine is not good for stability or compatibility - so I've been told. I've got no real experience of it as I've never mixed RAM sets before. So I'd go more for the RAM than the extra SSD. You can always easily add an extra SSD when you need the space later on. Unless of course you know you need the extra space now. But even then you could put your lesser used or smaller libraries onto an HDD for now.