Well, you must ONLY use Safari with ONE tab open.
I am currently listening to music, have 3 tabs open in safari, and when I open the Mac App store it shows that I'm using 2.72 GB of RAM.
Sure, 2Gb is enough, if you only check your e-mail or watch SD videos in Youtube.
That is not right, there is somethng wrong with your computer if you're using that much RAM for that.
Also worth noting that youtube uses flash, and upgrading your RAM won't affect the way it runs. Your CPU and video will be the drivers of that and whether it can adequately process SD or HD