Don't let me stop you from upgrading, that's got loads of upsides. Just suspecting you still have something else to fix before you'll really get to use it :)
It CAN be ok to have very low free ram if it's used up by buffers/cache. (freeable) If Buff/cache gets below about 3GB on most systems, you'll start to struggle.
If you have 16GB, it's running low, and you can't account for it in top, you have something leaking somewhere.