I don't know exactly to answer yes or no, but in my experience after a long run with 2GB RAM, it's enough for this moment if you don't do much multitasking.
Vista memory utilization are around 500-600MB. Let's say you play crysis which utilize around 500-600MB, means in total you are using 1-1.2GB RAM. The rest of 0.8-1GB are there, maybe there are antivirus, antispyware, background services and other softwares which are in total of them around 200-300MB. Well, in conclusion, around 500-700MB RAM are free all the time and if you have 4GB, you would see those 2 GB RAM will always free.
Different picture if you are multitasking such you burn CD, converting videos, converting photos, designing your house while playing games would give a boost if you have those 4GB RAM.