I want to get a VPS to run a mail server and some spam checking, mainly DSPAM and not much else.
What is the minimum specification for such a task.
Will a 128MB VPS be adequate?
Of course there are many variables to consider (such as # of users and mail volume) but I would say that 128MB should be adequate. Be sure to disable all services that you don't need. If you can get by without a control panel that will help also.
I use DSPAM personally on my mail server. It used to be great but its effectiveness has been reduced by the recent deluge of "image spam" emails. Now I use it in combination with SpamAssassin because it checks some things that a Bayesian-filter-only solution can't.