microapp
Check the RAM usage in the performance monitor and/or Task Manager to see if you are anywhere near 8GB.
Unless you are running several instances of Kontakt or some other sample based synths, I doubt you will even notice doubling the RAM.
+1 to the I7-4790K. Still the best bang for the buck. I built a 4790 PC for my friend's studio this summer and it rocks.
^^^This. More RAM will only help if it's a bottleneck now. Without using samplers, it's unlikely you're tapping out the 8 GB you already have, in which case adding more will do nothing for you. Sorry.
A faster CPU, OTOH, will allow more tracks and more effects before it runs out of steam, and video may be smoother.
However, the same principle applies as with memory: there's no advantage to having more than you need. It's not a video game or a sports car. If your i5 has no problem keeping up now, and you don't plan on starting significantly larger projects, then you may be disappointed by a CPU upgrade as well. Windows will boot faster, SONAR will come up faster, but you won't notice any difference in your recording projects.
It comes down to whether or not your current system is ever NOT up to the task, e.g. you're having to freeze tracks to avoid glitchy audio, you avoid certain plugins because they cause dropouts, or you want to watch YouTube videos while you're recording (not recommended).
Here's a thought: if you're looking for a cheap upgrade, consider adding a 1 TB external disk drive for backups. One you can disconnect and stash elsewhere in case your computer gets stolen.