Postgres on VPS - how much is enough?

How big a VPS would I need to run a Postgres DB.

I need a Postgres database with about 15 tables that will run on a
single virtual private server.

The 15 tables will be spread over three tablespaces (5 tables per
tablespace) and be accessed by three different applications running on
different machines.

One application will add about 500 orders per day
Another will access this data to create and send about 500 emails per day
A third will access this data to create an after-sales survey for at
most 500 times per day.

What type of VPS would I need to run a database with this type pf load?
Is 128 MB ram enough?
What percentage of a 2.8 GHz CPU would be required?

It's been a long time since I used Postgres.

Thanks for any help,
Nagita