The article seems very old. In the section on 'How much memory?' it quotes an advanced professional user getting by with 256MB of RAM though 512MB would be better! I think with todays PCs we are talking about GBs of RAM rather than MBs!
Generally the advantage is a speed boost - programs open faster and you can have more windows open running different programs at the same time, but, isn't there always a but, if your processor is not all that fast then it will create a bit of a bottleneck meaning that although you might have more than enough memory to run 3 or 4 programs at the same time the processor might not be able to cope slowing things down again.
A fast processor would not have this problem. Also there is a point where you will notice no real difference with an upgrade as only really memory intensive programs will show the advantage. By this I mean things like video editing and I am talking about upgrading from the likes of 1 gig of RAM to 2 gig.
Typical XP system with 256mb RAM, upgrade to 512mb and you would notice the difference with everything, quite a boost, go from 512mb to 1 gig and you would notice a speed boost with some things but not with others, go from 1 gig to 2 gig and in general the difference would be there but would be so small you probably would only notice it with memory intensive programs. Hope that helps.