It depends on which type of RAM you speak of.
I personally think that 2GB DDR2 is a minimum for Vista & 7 if you want a 'nice, smooth' experience.
So yes, i do think it's worth buying an extra 2GB (RAM is cheap as chips too). Please note that if you have a 32bit OS it won't address more than 3.2GB and the extra 1GB is useless. Also, make sure that your board has enough slots for the extra RAM