Whether you'll gain from a RAM upgrade depends on what your computing needs are. I've got a laptop with 2 GB installed, and it does 98% of what I do on my desktop machine with 4 GB -- but there are a couple programs I run (GIMP, Path of Exile) that would benefit, even on the desktop system, by upgrading RAM (unfortunately my MB is maxed with the 4 GB).
You may want to install a small CPU/RAM monitor of some kind so you can tell if performance issues are related to inadequate RAM, processor saturation, or possibly even video bottlenecks (the latter almost always in games or image/video editing). If your motherboard will support it RAM is the easiest upgrade, followed by video; some motherboards offer relatively easy CPU upgrades (PGA socket CPUs are a breeze, old-style sockets are a major pain, with lever-operated ZIF type in between), while others (mainly laptops) have the CPU soldered in and you'd have to replace the entire motherboard (or in a laptop the whole machine).
Overall, it's likely you'll see a very small increase in performance, if any, in most software going from 4 GB to 8 GB -- but if you play a game with high RAM demands or edit very large images or do a lot of video transcoding, doubling your RAM could be like a major system upgrade.