Your post is a bit confusing because you don't say what your current system specs are (other than the i5-760 cpu).
It sounds like you're saying you want to upgrade to Win 7-64 bit, 16GB of ram, and Sonar Platinum, is that correct?
16GB of ram will definitely help, but I doubt it will get you to where you can run multiple instances of Omnisphere. Not on your current cpu, and not without using freeze a lot. I honestly don't know if upping it to 32GB of ram would make a significant difference.
I can see why you'd want to skip Win 10, but why go to Win 7? Go to the better optimized Win 8.1 and use classic shell (which gets rid of the "metro" look so it looks like Win 7). Or are you saying you're already on Win 7 and don't want to spend more on the O.S.?
Installing one or two SSDs would help make things zippier/load faster, but here again your cpu will probably still be the bottleneck.