Adding 2GB to when you have just 2GB makes a difference. If you have like 16 and you add 16 then i dont think so, unless you're using ram heavy softwares like 3D apps or video editor.I've got 16gb of RAM and trust me it doesn't make any big difference. Having an SSD is way better.
That issue is solved. I added 2GB of RAM so now there is total 4GB.100% Disk usage when doing nothing can indicate problem with some programs or since you have 2GB ram, the disk is busy because is swaping files from ram to hdd when a program is idle and being used and vice versa ( Page file ). So i guess adding more ram will make a difference. I had similar problems since i had 2GB ram too. I add another 2GB and now my pc runs smooth even when using softwares like Photoshop and Illustrator.