Adding more RAM may or may not help.
You need to look at the memory tab of Resource Monitor when the system is sluggish. Unless all the memory is used up at that time (no blue in the graphs), then adding more memory isn't likely to make much difference.
You should look at your documentation or Acer's website to see the maximum RAM supported and the per-slot maximums for your model. You might be able to just replace the 1 GB stick with a 4 GB stick, giving you 6 total.
I'd think Windows 7 runs quite well on 3 GB, but I don't know exactly what load you are putting on it or how many tabs you have open.