I have a football forum which is currently using a shared hosting currently. I'm thinking of getting my first VPS (managed if possible).

Forum statistics below.

Forum Software: vBulletin
Unique Visitors: about 20,000 each month
Pageviews: about 280,000 pageviews each month

Visitors are mostly from South-East Asia (majority from Singapore).
We have an AJAX shoutbox on the homepage, and members frequently post and exchange pictures in the forum.

I'll be expanding the forum by end of year. Will be installing the new vB4 so that my writers and editors can publish football news daily.

How much RAM is enough to run this website? Any recommendations?