Didn't read through alot of the later suggestions, but defintely up your buffer beyond 128 for now to see check for improvement. You can also consider freezing your tracks for now, even if there are no fx running on them currently. Rather I mean you can render out what you have, and track in a new project with the backing track.
Your interface as per buffer settings will have an impact on your CPU. But then you have to start considering latency in the trade-off.
Even so though, with as few tracks as you have going on I'm surprised you're having an issue, especially with your given specs.
You will see a great improvement upgrading to an SSD, but unfortunately this shouldn't be causing the issue you are having with only the four odd tracks or so that you mentioned having, and when I say improvements I'm thinking general system performance. The issue is likely something else. I track and stream from 7.2kRPM drives all day long. Your HDD is negligible when considering how most samplers are loaded onto RAM, so long as you wait for them to load upon starting the project, this shouldn't be affecting the disk i/o a great deal...
RAM will only become a major factor if you find yourself running a large number of samplers that eat up resources. I've never noticed an improvement beyond avoiding crashing when loading projects too large from my RAM capacity. Even with the consideration of more samplers in the future, I would have to tell you that 8gb is enough (read: I have no idea what libraries you intend to use, or how you'll use them I often hit my ceiling of 12gb but most often because of the nature of the project!)
SSDs typically improves on just about everything you can imagine! I wouldn't want to use it for a scratch disk though, if I was limited to just the one disk. Wouldn't hurt either way. Again I think your issue lies elsewhere.
EDIT: Sorry, for some reason when you mentioned your issue, I assumed you meant 'snap, crackle, & pop' with your audio, typical of a low buffer or struggling CPU. Still given your specs, this is strange!