when I use MS word and excel 128MB more than enough, but when I do Finite Element Analysis with a model that have 50,000 elements and around 300,000 degrees of freedom to solve and wanting to calculate 450 mode shapes where my output is 500MB then I will need more than 512MB. usually RAM = output data * 5.
Because in between you will need headroom for rought calculations and storing temparaty answer before you get the complete answer.
All software does this.
If I don't have enough then Windows start to use Virtual memory which bogs down performance significantly.
Like all previous advice, what are you doing? thats how much you need.
Best regards
cx5