Which is better approach between holding object in memory or storing data in some file/database

I am working on a dashboard project which connects with two or three bug tracking system like jira, mantis etc.. and display graphical representation of data in web app.

What I am doing:

When any new project gets added, I connect and fetch data from these system and stores data in one table in database. After that there is web app which then process this data in table and displays charts and tables.

For updating, I have created a windows service which then continuously connect with bug tracking system and update table in database (Project configuration data is also stored in one another table)

Question:

1. What if I don't store bug tracking system data into database and store that data into object. Will create a web service which will fire update events upon update and will then refresh view?

2. What is right approach? Is it right to store whole project data into memory rather than database. What if there will be large number of project say 300 or 600?

3. How should I decide when to store data in memory and when in database?

4. Pros and cons of both ways?