A performance bump by increasing from 3gb to 4gb RAM?

If you just use the machine for light work and dont have many apps open at the same time then no, it wont make a huge difference. What would make a huge difference to the responsive of your iMac would be to swap the primary disk out for an SSD.

This of course would cost more and is a more challenging modification. The difference in performance however was like night and day. I followed a video guide from YouTube and it didn't cause me any sweat to be honest.

 

What do you mean - sometimes? In the morning?
What are you doing when it is slow? If you are short of RAM, the 1GB wouldn't do much, and if you are not - it wouldn't do anything.

 

Activity monitor.

even though the difference seems small, going from 'not enough' to 'enough' could make a huge difference (but then, going from 'not enough' to 'still not enough' wouldn't).

...It's hard to answer without actually trying, but I'm tempted to say that 4GB is worthwhile. It's what apple recommends for mountain lion I guess.

 

As pubjoe says, Activity Monitor is your friend.

1) I have the memory monitor on my dock at all times. That way I can easily see if a slowdown is RAM connected, or not. Note that the only colours that really matter are the Yellow and Red. Blue is RAM that was recently used, and the system is just waiting to see if the same application comes back and wants to use it again. However, if a different application needs RAM it will take it from the Blue, and become Yellow or Red. Green is good, as it is 'free' memory. If you are always showing Green on the memory monitor, even when your system is sluggish, then it's likely more RAM won't fix anything.

2) On the Activity Monitor Memory page, there is a stat for Page Outs. Do a search on that term here on MacRumours, for details... but basically - if your system is not "paging out' then more memory is not going to fix anything. The system uses Page Outs when it has insufficient RAM and is writing some of it to the hard-disk. If you are getting lots of Page Outs, then that will definitely slow you down. A lot.

#1 and #2 should be telling you kinda the same thing.

3) Activity Monitor can also show you how hard the CPU is working, and how much network traffic the system is creating. Get into the habit of checking these stats both when you are bogged down and when you aren't to get a feel for what is different.

4) Do you have at least 10% of your system HDD (hard-disk) free? If you are space starved on the HDD you can get slowdowns, and eventually perhaps a system lock up. OS X likes to have a chunk of free-space on the HDD to work smoothly.

5) Open up Disk Utility and 'Verify' the HDD. If there is a problem with the file system this will let you know, and then you can 'Repair' it... you'll need to boot from the installation disks or another bootable media to 'Repair'... but Verify will work fine and won't make any changes to the system... it just looks.

Good Luck.