Given the light usage, similar to what my dad does (but he has a Haswell era i3 and 4GB RAM), the amount of disk access Windows does made his system slow. Especially when booting then first logging in.
Once the first couple of minutes passed, it was fine. He'd check his financial websites, weather websites, do email, and watch YouTube videos. All using the Firefox browser.
Switching to an SSD was like night and day for him. He's still running on only 4GB of RAM, but I remember watching the RAM usage for what he does, and I don't think it ever exceeded 2.7GB.
So, getting a 2.5" SATA SSD is the way to go. As was described by others earlier
- back up any existing data
- download the Microsoft Windows creation tool (and have it create a bootable installer to USB)
- shut down
- remove the hard drive
- install the SSD
- boot to the USB stick and do a clean install of windows
- restore the data you backed up in step 1
That said, I think that, as software gets a little heavier, going to 8GB will help avoid swapping issues.
How many RAM slots does your system have? And, how is your current RAM set up/distributed:
- a single 4GB stick?
- a pair of 2GB sticks?
- four 1GB sticks?
- a pair of 1GB sticks plus a single 2GB stick?
Normally I'd say make sure they're matched, same speed, etc., but since you're not gaming, I don't know if that's going to matter much, so if you have some kind of odd mismatch (like the last one in my list), then your usage won't really suffer any noticeable performance irregularities (vs what might happen with gaming).
Ultimately, I'd say the priority is the SSD. Adding RAM might also help, but I'd do that as my second step rather than my first step.